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Diffstat (limited to 'Documentation/RCU/whatisRCU.txt')
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 29 |
1 files changed, 17 insertions, 12 deletions
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 15da16861fa..5ed85af8878 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -200,10 +200,11 @@ rcu_assign_pointer() 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(). + Perhaps just as important, it serves to document (1) which + pointers are protected by RCU and (2) the point at which a + given structure becomes accessible to other CPUs. That said, + rcu_assign_pointer() is most frequently used indirectly, via + the _rcu list-manipulation primitives such as list_add_rcu(). rcu_dereference() @@ -258,9 +259,11 @@ rcu_dereference() 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 + rcu_dereference() is to document which pointers are protected by + RCU, in particular, flagging a pointer that is subject to changing + at any time, including immediately after the rcu_dereference(). + 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 @@ -327,7 +330,7 @@ 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 +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 { @@ -410,6 +413,8 @@ o Use synchronize_rcu() -after- removing a data element from an data item. See checklist.txt for additional rules to follow when using RCU. +And again, more-typical uses of RCU may be found in listRCU.txt, +arrayRCU.txt, and NMI-RCU.txt. 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? @@ -513,7 +518,7 @@ production-quality implementation, and see: 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. +more details on the current implementation as of early 2004. 5A. "TOY" IMPLEMENTATION #1: LOCKING @@ -768,7 +773,6 @@ 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) @@ -807,7 +811,8 @@ Quick Quiz #1: Why is this argument naive? How could a deadlock Answer: Consider the following sequence of events: 1. CPU 0 acquires some unrelated lock, call it - "problematic_lock". + "problematic_lock", disabling irq via + spin_lock_irqsave(). 2. CPU 1 enters synchronize_rcu(), write-acquiring rcu_gp_mutex. @@ -894,7 +899,7 @@ Answer: Just as PREEMPT_RT permits preemption of spinlock ACKNOWLEDGEMENTS My thanks to the people who helped make this human-readable, including -Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood. +Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern. For more information, see http://www.rdrop.com/users/paulmck/RCU. |