diff options
| -rw-r--r-- | Documentation/RCU/RTFP.txt | 36 | ||||
| -rw-r--r-- | Documentation/RCU/UP.txt | 79 | ||||
| -rw-r--r-- | Documentation/RCU/checklist.txt | 23 | ||||
| -rw-r--r-- | Documentation/RCU/rcu.txt | 48 | ||||
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 902 |
5 files changed, 1064 insertions, 24 deletions
diff --git a/Documentation/RCU/RTFP.txt b/Documentation/RCU/RTFP.txt index 9c6d450138e..fcbcbc35b12 100644 --- a/Documentation/RCU/RTFP.txt +++ b/Documentation/RCU/RTFP.txt @@ -2,7 +2,8 @@ Read the F-ing Papers! This document describes RCU-related publications, and is followed by -the corresponding bibtex entries. +the corresponding bibtex entries. A number of the publications may +be found at http://www.rdrop.com/users/paulmck/RCU/. The first thing resembling RCU was published in 1980, when Kung and Lehman [Kung80] recommended use of a garbage collector to defer destruction @@ -113,6 +114,10 @@ describing how to make RCU safe for soft-realtime applications [Sarma04c], and a paper describing SELinux performance with RCU [JamesMorris04b]. +2005 has seen further adaptation of RCU to realtime use, permitting +preemption of RCU realtime critical sections [PaulMcKenney05a, +PaulMcKenney05b]. + Bibtex Entries @article{Kung80 @@ -410,3 +415,32 @@ Oregon Health and Sciences University" \url{http://www.livejournal.com/users/james_morris/2153.html} [Viewed December 10, 2004]" } + +@unpublished{PaulMcKenney05a +,Author="Paul E. McKenney" +,Title="{[RFC]} {RCU} and {CONFIG\_PREEMPT\_RT} progress" +,month="May" +,year="2005" +,note="Available: +\url{http://lkml.org/lkml/2005/5/9/185} +[Viewed May 13, 2005]" +,annotation=" + First publication of working lock-based deferred free patches + for the CONFIG_PREEMPT_RT environment. +" +} + +@conference{PaulMcKenney05b +,Author="Paul E. McKenney and Dipankar Sarma" +,Title="Towards Hard Realtime Response from the Linux Kernel on SMP Hardware" +,Booktitle="linux.conf.au 2005" +,month="April" +,year="2005" +,address="Canberra, Australia" +,note="Available: +\url{http://www.rdrop.com/users/paulmck/RCU/realtimeRCU.2005.04.23a.pdf} +[Viewed May 13, 2005]" +,annotation=" + Realtime turns into making RCU yet more realtime friendly. +" +} diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt index 3bfb84b3b7d..aab4a9ec393 100644 --- a/Documentation/RCU/UP.txt +++ b/Documentation/RCU/UP.txt @@ -8,7 +8,7 @@ is that since there is only one CPU, it should not be necessary to wait for anything else to get done, since there are no other CPUs for anything else to be happening on. Although this approach will -sort- -of- work a surprising amount of the time, it is a very bad idea in general. -This document presents two examples that demonstrate exactly how bad an +This document presents three examples that demonstrate exactly how bad an idea this is. @@ -26,6 +26,9 @@ from softirq, the list scan would find itself referencing a newly freed element B. This situation can greatly decrease the life expectancy of your kernel. +This same problem can occur if call_rcu() is invoked from a hardware +interrupt handler. + Example 2: Function-Call Fatality @@ -44,8 +47,37 @@ its arguments would cause it to fail to make the fundamental guarantee underlying RCU, namely that call_rcu() defers invoking its arguments until all RCU read-side critical sections currently executing have completed. -Quick Quiz: why is it -not- legal to invoke synchronize_rcu() in -this case? +Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in + this case? + + +Example 3: Death by Deadlock + +Suppose that call_rcu() is invoked while holding a lock, and that the +callback function must acquire this same lock. In this case, if +call_rcu() were to directly invoke the callback, the result would +be self-deadlock. + +In some cases, it would possible to restructure to code so that +the call_rcu() is delayed until after the lock is released. However, +there are cases where this can be quite ugly: + +1. If a number of items need to be passed to call_rcu() within + the same critical section, then the code would need to create + a list of them, then traverse the list once the lock was + released. + +2. In some cases, the lock will be held across some kernel API, + so that delaying the call_rcu() until the lock is released + requires that the data item be passed up via a common API. + It is far better to guarantee that callbacks are invoked + with no locks held than to have to modify such APIs to allow + arbitrary data items to be passed back up through them. + +If call_rcu() directly invokes the callback, painful locking restrictions +or API changes would be required. + +Quick Quiz #2: What locking restriction must RCU callbacks respect? Summary @@ -53,12 +85,35 @@ Summary Permitting call_rcu() to immediately invoke its arguments or permitting synchronize_rcu() to immediately return breaks RCU, even on a UP system. So do not do it! Even on a UP system, the RCU infrastructure -must- -respect grace periods. - - -Answer to Quick Quiz - -The calling function is scanning an RCU-protected linked list, and -is therefore within an RCU read-side critical section. Therefore, -the called function has been invoked within an RCU read-side critical -section, and is not permitted to block. +respect grace periods, and -must- invoke callbacks from a known environment +in which no locks are held. + + +Answer to Quick Quiz #1: + Why is it -not- legal to invoke synchronize_rcu() in this case? + + Because the calling function is scanning an RCU-protected linked + list, and is therefore within an RCU read-side critical section. + Therefore, the called function has been invoked within an RCU + read-side critical section, and is not permitted to block. + +Answer to Quick Quiz #2: + What locking restriction must RCU callbacks respect? + + Any lock that is acquired within an RCU callback must be + acquired elsewhere using an _irq variant of the spinlock + primitive. For example, if "mylock" is acquired by an + RCU callback, then a process-context acquisition of this + lock must use something like spin_lock_irqsave() to + acquire the lock. + + If the process-context code were to simply use spin_lock(), + then, since RCU callbacks can be invoked from softirq context, + the callback might be called from a softirq that interrupted + the process-context critical section. This would result in + self-deadlock. + + This restriction might seem gratuitous, since very few RCU + callbacks acquire locks directly. However, a great many RCU + callbacks do acquire locks -indirectly-, for example, via + the kfree() primitive. diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 8f3fb77c9cd..e118a7c1a09 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -43,6 +43,10 @@ over a rather long period of time, but improvements are always welcome! rcu_read_lock_bh()) in the read-side critical sections, and are also an excellent aid to readability. + As a rough rule of thumb, any dereference of an RCU-protected + pointer must be covered by rcu_read_lock() or rcu_read_lock_bh() + or by the appropriate update-side lock. + 3. Does the update code tolerate concurrent accesses? The whole point of RCU is to permit readers to run without @@ -90,7 +94,11 @@ over a rather long period of time, but improvements are always welcome! The rcu_dereference() primitive is used by the various "_rcu()" list-traversal primitives, such as the - list_for_each_entry_rcu(). + list_for_each_entry_rcu(). Note that it is perfectly + legal (if redundant) for update-side code to use + rcu_dereference() and the "_rcu()" list-traversal + primitives. This is particularly useful in code + that is common to readers and updaters. b. If the list macros are being used, the list_add_tail_rcu() and list_add_rcu() primitives must be used in order @@ -150,16 +158,9 @@ over a rather long period of time, but improvements are always welcome! Use of the _rcu() list-traversal primitives outside of an RCU read-side critical section causes no harm other than - a slight performance degradation on Alpha CPUs and some - confusion on the part of people trying to read the code. - - Another way of thinking of this is "If you are holding the - lock that prevents the data structure from changing, why do - you also need RCU-based protection?" That said, there may - well be situations where use of the _rcu() list-traversal - primitives while the update-side lock is held results in - simpler and more maintainable code. The jury is still out - on this question. + a slight performance degradation on Alpha CPUs. It can + also be quite helpful in reducing code bloat when common + code is shared between readers and updaters. 10. Conversely, if you are in an RCU read-side critical section, you -must- use the "_rcu()" variants of the list macros. diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt index eb444006683..6fa09225158 100644 --- a/Documentation/RCU/rcu.txt +++ b/Documentation/RCU/rcu.txt @@ -64,6 +64,54 @@ o I hear that RCU is patented? What is with that? Of these, one was allowed to lapse by the assignee, and the others have been contributed to the Linux kernel under GPL. +o I hear that RCU needs work in order to support realtime kernels? + + Yes, work in progress. + o Where can I find more information on RCU? See the RTFP.txt file in this directory. + Or point your browser at http://www.rdrop.com/users/paulmck/RCU/. + +o What are all these files in this directory? + + + NMI-RCU.txt + + Describes how to use RCU to implement dynamic + NMI handlers, which can be revectored on the fly, + without rebooting. + + RTFP.txt + + List of RCU-related publications and web sites. + + UP.txt + + Discussion of RCU usage in UP kernels. + + arrayRCU.txt + + Describes how to use RCU to protect arrays, with + resizeable arrays whose elements reference other + data structures being of the most interest. + + checklist.txt + + Lists things to check for when inspecting code that + uses RCU. + + listRCU.txt + + Describes how to use RCU to protect linked lists. + This is the simplest and most common use of RCU + in the Linux kernel. + + rcu.txt + + You are reading it! + + whatisRCU.txt + + Overview of how the RCU implementation works. Along + the way, presents a conceptual view of RCU. diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt new file mode 100644 index 00000000000..354d89c7837 --- /dev/null +++ b/Documentation/RCU/whatisRCU.txt @@ -0,0 +1,902 @@ +What is RCU? + +RCU is a synchronization mechanism that was added to the Linux kernel +during the 2.5 development effort that is optimized for read-mostly +situations. Although RCU is actually quite simple once you understand it, +getting there can sometimes be a challenge. Part of the problem is that +most of the past descriptions of RCU have been written with the mistaken +assumption that there is "one true way" to describe RCU. Instead, +the experience has been that different people must take different paths +to arrive at an understanding of RCU. This document provides several +different paths, as follows: + +1. RCU OVERVIEW +2. WHAT IS RCU'S CORE API? +3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? +4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? +5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? +6. ANALOGY WITH READER-WRITER LOCKING +7. FULL LIST OF RCU APIs +8. ANSWERS TO QUICK QUIZZES + +People who prefer starting with a conceptual overview should focus on +Section 1, though most readers will profit by reading this section at +some point. People who prefer to start with an API that they can then +experiment with should focus on Section 2. People who prefer to start +with example uses should focus on Sections 3 and 4. People who need to +understand the RCU implementation should focus on Section 5, then dive +into the kernel source code. People who reason best by analogy should +focus on Section 6. Section 7 serves as an index to the docbook API +documentation, and Section 8 is the traditional answer key. + +So, start with the section that makes the most sense to you and your +preferred method of learning. If you need to know everything about +everything, feel free to read the whole thing -- but if you are really +that type of person, you have perused the source code and will therefore +never need this document anyway. ;-) + + +1. RCU OVERVIEW + +The basic idea behind RCU is to split updates into "removal" and +"reclamation" phases. The removal phase removes references to data items +within a data structure (possibly by replacing them with references to +new versions of these data items), and can run concurrently with readers. +The reason that it is safe to run the removal phase concurrently with +readers is the semantics of modern CPUs guarantee that readers will see +either the old or the new version of the data structure rather than a +partially updated reference. The reclamation phase does the work of reclaiming +(e.g., freeing) the data items removed from the data structure during the +removal phase. Because reclaiming data items can disrupt any readers +concurrently referencing those data items, the reclamation phase must +not start until readers no longer hold references to those data items. + +Splitting the update into removal and reclamation phases permits the +updater to perform the removal phase immediately, and to defer the +reclamation phase until all readers active during the removal phase have +completed, either by blocking until they finish or by registering a +callback that is invoked after they finish. Only readers that are active +during the removal phase need be considered, because any reader starting +after the removal phase will be unable to gain a reference to the removed +data items, and therefore cannot be disrupted by the reclamation phase. + +So the typical RCU update sequence goes something like the following: + +a. Remove pointers to a data structure, so that subsequent + readers cannot gain a reference to it. + +b. Wait for all previous readers to complete their RCU read-side + critical sections. + +c. At this point, there cannot be any readers who hold references + to the data structure, so it now may safely be reclaimed + (e.g., kfree()d). + +Step (b) above is the key idea underlying RCU's deferred destruction. +The ability to wait until all readers are done allows RCU readers to +use much lighter-weight synchronization, in some cases, absolutely no +synchronization at all. In contrast, in more conventional lock-based +schemes, readers must use heavy-weight synchronization in order to +prevent an updater from deleting the data structure out from under them. +This is because lock-based updaters typically update data items in place, +and must therefore exclude readers. In contrast, RCU-based updaters +typically take advantage of the fact that writes to single aligned +pointers are atomic on modern CPUs, allowing atomic insertion, removal, +and replacement of data items in a linked structure without disrupting +readers. Concurrent RCU readers can then continue accessing the old +versions, and can dispense with the atomic operations, memory barriers, +and communications cache misses that are so expensive on present-day +SMP computer systems, even in absence of lock contention. + +In the three-step procedure shown above, the updater is performing both +the removal and the reclamation step, but it is often helpful for an +entirely different thread to do the reclamation, as is in fact the case +in the Linux kernel's directory-entry cache (dcache). Even if the same +thread performs both the update step (step (a) above) and the reclamation +step (step (c) above), it is often helpful to think of them separately. +For example, RCU readers and updaters need not communicate at all, +but RCU provides implicit low-overhead communication between readers +and reclaimers, namely, in step (b) above. + +So how the heck can a reclaimer tell when a reader is done, given +that readers are not doing any sort of synchronization operations??? +Read on to learn about how RCU's API makes this easy. + + +2. WHAT IS RCU'S CORE API? + +The core RCU API is quite small: + +a. rcu_read_lock() +b. rcu_read_unlock() +c. synchronize_rcu() / call_rcu() +d. rcu_assign_pointer() +e. rcu_dereference() + +There are many other members of the RCU API, but the rest can be +expressed in terms of these five, though most implementations instead +express synchronize_rcu() in terms of the call_rcu() callback API. + +The five core RCU APIs are described below, the other 18 will be enumerated +later. See the kernel docbook documentation for more info, or look directly +at the function header comments. + +rcu_read_lock() + + void rcu_read_lock(void); + + Used by a reader to inform the reclaimer that the reader is + entering an RCU read-side critical section. It is illegal + to block while in an RCU read-side critical section, though + kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side + critical sections. Any RCU-protected data structure accessed + during an RCU read-side critical section is guaranteed to remain + unreclaimed for the full duration of that critical section. + Reference counts may be used in conjunction with RCU to maintain + longer-term references to data structures. + +rcu_read_unlock() + + void rcu_read_unlock(void); + + Used by a reader to inform the reclaimer that the reader is + exiting an RCU read-side critical section. Note that RCU + read-side critical sections may be nested and/or overlapping. + +synchronize_rcu() + + void synchronize_rcu(void); + + Marks the end of updater code and the beginning of reclaimer + code. It does this by blocking until all pre-existing RCU + read-side critical sections on all CPUs have completed. + Note that synchronize_rcu() will -not- necessarily wait for + any subsequent RCU read-side critical sections to complete. + For example, consider the following sequence of events: + + CPU 0 CPU 1 CPU 2 + ----------------- ------------------------- --------------- + 1. rcu_read_lock() + 2. enters synchronize_rcu() + 3. rcu_read_lock() + 4. rcu_read_unlock() + 5. exits synchronize_rcu() + 6. rcu_read_unlock() + + To reiterate, synchronize_rcu() waits only for ongoing RCU + read-side critical sections to complete, not necessarily for + any that begin after synchronize_rcu() is invoked. + + Of course, synchronize_rcu() does not necessarily return + -immediately- after the last pre-existing RCU read-side critical + section completes. For one thing, there might well be scheduling + delays. For another thing, many RCU implementations process + requests in batches in order to improve efficiencies, which can + further delay synchronize_rcu(). + + Since synchronize_rcu() is the API that must figure out when + readers are done, its implementation is key to RCU. For RCU + to be useful in all but the most read-intensive situations, + synchronize_rcu()'s overhead must also be quite small. + + The call_rcu() API is a callback form of synchronize_rcu(), + and is described in more detail in a later section. Instead of + blocking, it registers a function and argument which are invoked + after all ongoing RCU read-side critical sections have completed. + This callback variant is particularly useful in situations where + it is illegal to block. + +rcu_assign_pointer() + + typeof(p) rcu_assign_pointer(p, typeof(p) v); + + Yes, rcu_assign_pointer() -is- implemented as a macro, though it + would be cool to be able to declare a function in this manner. + (Compiler experts will no doubt disagree.) + + The updater uses this function to assign a new value to an + RCU-protected pointer, in order to safely communicate the change + in value from the updater to the reader. This function returns + the new value, and also executes any memory-barrier instructions + required for a given CPU architecture. + + Perhaps more important, it serves to document which pointers + are protected by RCU. That said, rcu_assign_pointer() is most + frequently used indirectly, via the _rcu list-manipulation + primitives such as list_add_rcu(). + +rcu_dereference() + + typeof(p) rcu_dereference(p); + + Like rcu_assign_pointer(), rcu_dereference() must be implemented + as a macro. + + The reader uses rcu_dereference() to fetch an RCU-protected + pointer, which returns a value that may then be safely + dereferenced. Note that rcu_deference() does not actually + dereference the pointer, instead, it protects the pointer for + later dereferencing. It also executes any needed memory-barrier + instructions for a given CPU architecture. Currently, only Alpha + needs memory barriers within rcu_dereference() -- on other CPUs, + it compiles to nothing, not even a compiler directive. + + Common coding practice uses rcu_dereference() to copy an + RCU-protected pointer to a local variable, then dereferences + this local variable, for example as follows: + + p = rcu_dereference(head.next); + return p->data; + + However, in this case, one could just as easily combine these + into one statement: + + return rcu_dereference(head.next)->data; + + If you are going to be fetching multiple fields from the + RCU-protected structure, using the local variable is of + course preferred. Repeated rcu_dereference() calls look + ugly and incur unnecessary overhead on Alpha CPUs. + + Note that the value returned by rcu_dereference() is valid + only within the enclosing RCU read-side critical section. + For example, the following is -not- legal: + + rcu_read_lock(); + p = rcu_dereference(head.next); + rcu_read_unlock(); + x = p->address; + rcu_read_lock(); + y = p->data; + rcu_read_unlock(); + + Holding a reference from one RCU read-side critical section + to another is just as illegal as holding a reference from + one lock-based critical section to another! Similarly, + using a reference outside of the critical section in which + it was acquired is just as illegal as doing so with normal + locking. + + As with rcu_assign_pointer(), an important function of + rcu_dereference() is to document which pointers are protected + by RCU. And, again like rcu_assign_pointer(), rcu_dereference() + is typically used indirectly, via the _rcu list-manipulation + primitives, such as list_for_each_entry_rcu(). + +The following diagram shows how each API communicates among the +reader, updater, and reclaimer. + + + rcu_assign_pointer() + +--------+ + +---------------------->| reader |---------+ + | +--------+ | + | | | + | | | Protect: + | | | rcu_read_lock() + | | | rcu_read_unlock() + | rcu_dereference() | | + +---------+ | | + | updater |<---------------------+ | + +---------+ V + | +-----------+ + +----------------------------------->| reclaimer | + +-----------+ + Defer: + synchronize_rcu() & call_rcu() + + +The RCU infrastructure observes the time sequence of rcu_read_lock(), +rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in +order to determine when (1) synchronize_rcu() invocations may return +to their callers and (2) call_rcu() callbacks may be invoked. Efficient +implementations of the RCU infrastructure make heavy use of batching in +order to amortize their overhead over many uses of the corresponding APIs. + +There are no fewer than three RCU mechanisms in the Linux kernel; the +diagram above shows the first one, which is by far the most commonly used. +The rcu_dereference() and rcu_assign_pointer() primitives are used for +all three mechanisms, but different defer and protect primitives are +used as follows: + + Defer Protect + +a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock() + call_rcu() + +b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh() + +c. synchronize_sched() preempt_disable() / preempt_enable() + local_irq_save() / local_irq_restore() + hardirq enter / hardirq exit + NMI enter / NMI exit + +These three mechanisms are used as follows: + +a. RCU applied to normal data structures. + +b. RCU applied to networking data structures that may be subjected + to remote denial-of-service attacks. + +c. RCU applied to scheduler and interrupt/NMI-handler tasks. + +Again, most uses will be of (a). The (b) and (c) cases are important +for specialized uses, but are relatively uncommon. + + +3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? + +This section shows a simple use of the core RCU API to protect a +global pointer to a dynamically allocated structure. More typical +uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. + + struct foo { + int a; + char b; + long c; + }; + DEFINE_SPINLOCK(foo_mutex); + + struct foo *gbl_foo; + + /* + * Create a new struct foo that is the same as the one currently + * pointed to by gbl_foo, except that field "a" is replaced + * with "new_a". Points gbl_foo to the new structure, and + * frees up the old structure after a grace period. + * + * Uses rcu_assign_pointer() to ensure that concurrent readers + * see the initialized version of the new structure. + * + * Uses synchronize_rcu() to ensure that any readers that might + * have references to the old structure complete before freeing + * the old structure. + */ + void foo_update_a(int new_a) + { + struct foo *new_fp; + struct foo *old_fp; + + new_fp = kmalloc(sizeof(*fp), GFP_KERNEL); + spin_lock(&foo_mutex); + old_fp = gbl_foo; + *new_fp = *old_fp; + new_fp->a = new_a; + rcu_assign_pointer(gbl_foo, new_fp); + spin_unlock(&foo_mutex); + synchronize_rcu(); + kfree(old_fp); + } + + /* + * Return the value of field "a" of the current gbl_foo + * structure. Use rcu_read_lock() and rcu_read_unlock() + * to ensure that the structure does not get deleted out + * from under us, and use rcu_dereference() to ensure that + * we see the initialized version of the structure (important + * for DEC Alpha and for people reading the code). + */ + int foo_get_a(void) + { + int retval; + + rcu_read_lock(); + retval = rcu_dereference(gbl_foo)->a; + rcu_read_unlock(); + return retval; + } + +So, to sum up: + +o Use rcu_read_lock() and rcu_read_unlock() to guard RCU + read-side critical sections. + +o Within an RCU read-side critical section, use rcu_dereference() + to dereference RCU-protected pointers. + +o Use some solid scheme (such as locks or semaphores) to + keep concurrent updates from interfering with each other. + +o Use rcu_assign_pointer() to update an RCU-protected pointer. + This primitive protects concurrent readers from the updater, + -not- concurrent updates from each other! You therefore still + need to use locking (or something similar) to keep concurrent + rcu_assign_pointer() primitives from interfering with each other. + +o Use synchronize_rcu() -after- removing a data element from an + RCU-protected data structure, but -before- reclaiming/freeing + the data element, in order to wait for the completion of all + RCU read-side critical sections that might be referencing that + data item. + +See checklist.txt for additional rules to follow when using RCU. + + +4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? + +In the example above, foo_update_a() blocks until a grace period elapses. +This is quite simple, but in some cases one cannot afford to wait so +long -- there might be other high-priority work to be done. + +In such cases, one uses call_rcu() rather than synchronize_rcu(). +The call_rcu() API is as follows: + + void call_rcu(struct rcu_head * head, + void (*func)(struct rcu_head *head)); + +This function invokes func(head) after a grace period has elapsed. +This invocation might happen from either softirq or process context, +so the function is not permitted to block. The foo struct needs to +have an rcu_head structure added, perhaps as follows: + + struct foo { + int a; + char b; + long c; + struct rcu_head rcu; + }; + +The foo_update_a() function might then be written as follows: + + /* + * Create a new struct foo that is the same as the one currently + * pointed to by gbl_foo, except that field "a" is replaced + * with "new_a". Points gbl_foo to the new structure, and + * frees up the old structure after a grace period. + * + * Uses rcu_assign_pointer() to ensure that concurrent readers + * see the initialized version of the new structure. + * + * Uses call_rcu() to ensure that any readers that might have + * references to the old structure complete before freeing the + * old structure. + */ + void foo_update_a(int new_a) + { + struct foo *new_fp; + struct foo *old_fp; + + new_fp = kmalloc(sizeof(*fp), GFP_KERNEL); + spin_lock(&foo_mutex); + old_fp = gbl_foo; + *new_fp = *old_fp; + new_fp->a = new_a; + rcu_assign_pointer(gbl_foo, new_fp); + spin_unlock(&foo_mutex); + call_rcu(&old_fp->rcu, foo_reclaim); + } + +The foo_reclaim() function might appear as follows: + + void foo_reclaim(struct rcu_head *rp) + { + struct foo *fp = container_of(rp, struct foo, rcu); + + kfree(fp); + } + +The container_of() primitive is a macro that, given a pointer into a +struct, the type of the struct, and the pointed-to field within the +struct, returns a pointer to the beginning of the struct. + +The use of call_rcu() permits the caller of foo_update_a() to +immediately regain control, without needing to worry further about the +old version of the newly updated element. It also clearly shows the +RCU distinction between updater, namely foo_update_a(), and reclaimer, +namely foo_reclaim(). + +The summary of advice is the same as for the previous section, except +that we are now using call_rcu() rather than synchronize_rcu(): + +o Use call_rcu() -after- removing a data element from an + RCU-protected data structure in order to register a callback + function that will be invoked after the completion of all RCU + read-side critical sections that might be referencing that + data item. + +Again, see checklist.txt for additional rules governing the use of RCU. + + +5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? + +One of the nice things about RCU is that it has extremely simple "toy" +implementations that are a good first step towards understanding the +production-quality implementations in the Linux kernel. This section +presents two such "toy" implementations of RCU, one that is implemented +in terms of familiar locking primitives, and another that more closely +resembles "classic" RCU. Both are way too simple for real-world use, +lacking both functionality and performance. However, they are useful +in getting a feel for how RCU works. See kernel/rcupdate.c for a +production-quality implementation, and see: + + http://www.rdrop.com/users/paulmck/RCU + +for papers describing the Linux kernel RCU implementation. The OLS'01 +and OLS'02 papers are a good introduction, and the dissertation provides +more details on the current implementation. + + +5A. "TOY" IMPLEMENTATION #1: LOCKING + +This section presents a "toy" RCU implementation that is based on +familiar locking primitives. Its overhead makes it a non-starter for +real-life use, as does its lack of scalability. It is also unsuitable +for realtime use, since it allows scheduling latency to "bleed" from +one read-side critical section to another. + +However, it is probably the easiest implementation to relate to, so is +a good starting point. + +It is extremely simple: + + static DEFINE_RWLOCK(rcu_gp_mutex); + + void rcu_read_lock(void) + { + read_lock(&rcu_gp_mutex); + } + + void rcu_read_unlock(void) + { + read_unlock(&rcu_gp_mutex); + } + + void synchronize_rcu(void) + { + write_lock(&rcu_gp_mutex); + write_unlock(&rcu_gp_mutex); + } + +[You can ignore rcu_assign_pointer() and rcu_dereference() without +missing much. But here they are anyway. And whatever you do, don't +forget about them when submitting patches making use of RCU!] + + #define rcu_assign_pointer(p, v) ({ \ + smp_wmb(); \ + (p) = (v); \ + }) + + #define rcu_dereference(p) ({ \ + typeof(p) _________p1 = p; \ + smp_read_barrier_depends(); \ + (_________p1); \ + }) + + +The rcu_read_lock() and rcu_read_unlock() primitive read-acquire +and release a global reader-writer lock. The synchronize_rcu() +primitive write-acquires this same lock, then immediately releases +it. This means that once synchronize_rcu() exits, all RCU read-side +critical sections that were in progress before synchonize_rcu() was +called are guaranteed to have completed -- there is no way that +synchronize_rcu() would have been able to write-acquire the lock +otherwise. + +It is possible to nest rcu_read_lock(), since reader-writer locks may +be recursively acquired. Note also that rcu_read_lock() is immune +from deadlock (an important property of RCU). The reason for this is +that the only thing that can block rcu_read_lock() is a synchronize_rcu(). +But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex, +so there can be no deadlock cycle. + +Quick Quiz #1: Why is this argument naive? How could a deadlock + occur when using this algorithm in a real-world Linux + kernel? How could this deadlock be avoided? + + +5B. "TOY" EXAMPLE #2: CLASSIC RCU + +This section presents a "toy" RCU implementation that is based on +"classic RCU". It is also short on performance (but only for updates) and +on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT +kernels. The definitions of rcu_dereference() and rcu_assign_pointer() +are the same as those shown in the preceding section, so they are omitted. + + void rcu_read_lock(void) { } + + void rcu_read_unlock(void) { } + + void synchronize_rcu(void) + { + int cpu; + + for_each_cpu(cpu) + run_on(cpu); + } + +Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing. +This is the great strength of classic RCU in a non-preemptive kernel: +read-side overhead is precisely zero, at least on non-Alpha CPUs. +And there is absolutely no way that rcu_read_lock() can possibly +participate in a deadlock cycle! + +The implementation of synchronize_rcu() simply schedules itself on each +CPU in turn. The run_on() primitive can be implemented straightforwardly +in terms of the sched_setaffinity() primitive. Of course, a somewhat less +"toy" implementation would restore the affinity upon completion rather +than just leaving all tasks running on the last CPU, but when I said +"toy", I meant -toy-! + +So how the heck is this supposed to work??? + +Remember that it is illegal to block while in an RCU read-side critical +section. Therefore, if a given CPU executes a context switch, we know +that it must have completed all preceding RCU read-side critical sections. +Once -all- CPUs have executed a context switch, then -all- preceding +RCU read-side critical sections will have completed. + +So, suppose that we remove a data item from its structure and then invoke +synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed +that there are no RCU read-side critical sections holding a reference +to that data item, so we can safely reclaim it. + +Quick Quiz #2: Give an example where Classic RCU's read-side + overhead is -negative-. + +Quick Quiz #3: If it is illegal to block in an RCU read-side + critical section, what the heck do you do in + PREEMPT_RT, where normal spinlocks can block??? + + +6. ANALOGY WITH READER-WRITER LOCKING + +Although RCU can be used in many different ways, a very common use of +RCU is analogous to reader-writer locking. The following unified +diff shows how closely related RCU and reader-writer locking can be. + + @@ -13,15 +14,15 @@ + struct list_head *lp; + struct el *p; + + - read_lock(); + - list_for_each_entry(p, head, lp) { + + rcu_read_lock(); + + list_for_each_entry_rcu(p, head, lp) { + if (p->key == key) { + *result = p->data; + - read_unlock(); + + rcu_read_unlock(); + return 1; + } + } + - read_unlock(); + + rcu_read_unlock(); + return 0; + } + + @@ -29,15 +30,16 @@ + { + struct el *p; + + - write_lock(&listmutex); + + spin_lock(&listmutex); + list_for_each_entry(p, head, lp) { + if (p->key == key) { + list_del(&p->list); + - write_unlock(&listmutex); + + spin_unlock(&listmutex); + + synchronize_rcu(); + kfree(p); + return 1; + } + } + - write_unlock(&listmutex); + + spin_unlock(&listmutex); + return 0; + } + +Or, for those who prefer a side-by-side listing: + + 1 struct el { 1 struct el { + 2 struct list_head list; 2 struct list_head list; + 3 long key; 3 long key; + 4 spinlock_t mutex; 4 spinlock_t mutex; + 5 int data; 5 int data; + 6 /* Other data fields */ 6 /* Other data fields */ + 7 }; 7 }; + 8 spinlock_t listmutex; 8 spinlock_t listmutex; + 9 struct el head; 9 struct el head; + + 1 int search(long key, int *result) 1 int search(long key, int *result) + 2 { 2 { + 3 struct list_head *lp; 3 struct list_head *lp; + 4 struct el *p; 4 struct el *p; + 5 5 + 6 read_lock(); 6 rcu_read_lock(); + 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { + 8 if (p->key == key) { 8 if (p->key == key) { + 9 *result = p->data; 9 *result = p->data; +10 read_unlock(); 10 rcu_read_unlock(); +11 return 1; 11 return 1; +12 } 12 } +13 } 13 } +14 read_unlock(); 14 rcu_read_unlock(); +15 return 0; 15 return 0; +16 } 16 } + + 1 int delete(long key) 1 int delete(long key) + 2 { 2 { + 3 struct el *p; 3 struct el *p; + 4 4 + 5 write_lock(&listmutex); 5 spin_lock(&listmutex); + 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { + 7 if (p->key == key) { 7 if (p->key == key) { + 8 list_del(&p->list); 8 list_del(&p->list); + 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); + 10 synchronize_rcu(); +10 kfree(p); 11 kfree(p); +11 return 1; 12 return 1; +12 } 13 } +13 } 14 } +14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); +15 return 0; 16 return 0; +16 } 17 } + +Either way, the differences are quite small. Read-side locking moves +to rcu_read_lock() and rcu_read_unlock, update-side locking moves from +from a reader-writer lock to a simple spinlock, and a synchronize_rcu() +precedes the kfree(). + +However, there is one potential catch: the read-side and update-side +critical sections can now run concurrently. In many cases, this will +not be a problem, but it is necessary to check carefully regardless. +For example, if multiple independent list updates must be seen as +a single atomic update, converting to RCU will require special care. + +Also, the presence of synchronize_rcu() means that the RCU version of +delete() can now block. If this is a problem, there is a callback-based +mechanism that never blocks, namely call_rcu(), that can be used in +place of synchronize_rcu(). + + +7. FULL LIST OF RCU APIs + +The RCU APIs are documented in docbook-format header comments in the +Linux-kernel source code, but it helps to have a full list of the +APIs, since there does not appear to be a way to categorize them +in docbook. Here is the list, by category. + +Markers for RCU read-side critical sections: + + rcu_read_lock + rcu_read_unlock + rcu_read_lock_bh + rcu_read_unlock_bh + +RCU pointer/list traversal: + + rcu_dereference + list_for_each_rcu (to be deprecated in favor of + list_for_each_entry_rcu) + list_for_each_safe_rcu (deprecated, not used) + list_for_each_entry_rcu + list_for_each_continue_rcu (to be deprecated in favor of new + list_for_each_entry_continue_rcu) + hlist_for_each_rcu (to be deprecated in favor of + hlist_for_each_entry_rcu) + hlist_for_each_entry_rcu + +RCU pointer update: + + rcu_assign_pointer + list_add_rcu + list_add_tail_rcu + list_del_rcu + list_replace_rcu + hlist_del_rcu + hlist_add_head_rcu + +RCU grace period: + + synchronize_kernel (deprecated) + synchronize_net + synchronize_sched + synchronize_rcu + call_rcu + call_rcu_bh + +See the comment headers in the source code (or the docbook generated +from them) for more information. + + +8. ANSWERS TO QUICK QUIZZES + +Quick Quiz #1: Why is this argument naive? How could a deadlock + occur when using this algorithm in a real-world Linux + kernel? [Referring to the lock-based "toy" RCU + algorithm.] + +Answer: Consider the following sequence of events: + + 1. CPU 0 acquires some unrelated lock, call it + "problematic_lock". + + 2. CPU 1 enters synchronize_rcu(), write-acquiring + rcu_gp_mutex. + + 3. CPU 0 enters rcu_read_lock(), but must wait + because CPU 1 holds rcu_gp_mutex. + + 4. CPU 1 is interrupted, and the irq handler + attempts to acquire problematic_lock. + + The system is now deadlocked. + + One way to avoid this deadlock is to use an approach like + that of CONFIG_PREEMPT_RT, where all normal spinlocks + become blocking locks, and all irq handlers execute in + the context of special tasks. In this case, in step 4 + above, the irq handler would block, allowing CPU 1 to + release rcu_gp_mutex, avoiding the deadlock. + + Even in the absence of deadlock, this RCU implementation + allows latency to "bleed" from readers to other + readers through synchronize_rcu(). To see this, + consider task A in an RCU read-side critical section + (thus read-holding rcu_gp_mutex), task B blocked + attempting to write-acquire rcu_gp_mutex, and + task C blocked in rcu_read_lock() attempting to + read_acquire rcu_gp_mutex. Task A's RCU read-side + latency is holding up task C, albeit indirectly via + task B. + + Realtime RCU implementations therefore use a counter-based + approach where tasks in RCU read-side critical sections + cannot be blocked by tasks executing synchronize_rcu(). + +Quick Quiz #2: Give an example where Classic RCU's read-side + overhead is -negative-. + +Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT + kernel where a routing table is used by process-context + code, but can be updated by irq-context code (for example, + by an "ICMP REDIRECT" packet). The usual way of handling + this would be to have the process-context code disable + interrupts while searching the routing table. Use of + RCU allows such interrupt-disabling to be dispensed with. + Thus, without RCU, you pay the cost of disabling interrupts, + and with RCU you don't. + + One can argue that the overhead of RCU in this + case is negative with respect to the single-CPU + interrupt-disabling approach. Others might argue that + the overhead of RCU is merely zero, and that replacing + the positive overhead of the interrupt-disabling scheme + with the zero-overhead RCU scheme does not constitute + negative overhead. + + In real life, of course, things are more complex. But + even the theoretical possibility of negative overhead for + a synchronization primitive is a bit unexpected. ;-) + +Quick Quiz #3: If it is illegal to block in an RCU read-side + critical section, what the heck do you do in + PREEMPT_RT, where normal spinlocks can block??? + +Answer: Just as PREEMPT_RT permits preemption of spinlock + critical sections, it permits preemption of RCU + read-side critical sections. It also permits + spinlocks blocking while in RCU read-side critical + sections. + + Why the apparent inconsistency? Because it is it + possible to use priority boosting to keep the RCU + grace periods short if need be (for example, if running + short of memory). In contrast, if blocking waiting + for (say) network reception, there is no way to know + what should be boosted. Especially given that the + process we need to boost might well be a human being + who just went out for a pizza or something. And although + a computer-operated cattle prod might arouse serious + interest, it might also provoke serious objections. + Besides, how does the computer know what pizza parlor + the human being went to??? + + +ACKNOWLEDGEMENTS + +My thanks to the people who helped make this human-readable, including +Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood. + + +For more information, see http://www.rdrop.com/users/paulmck/RCU. |