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| -rw-r--r-- | Documentation/RCU/rculist_nulls.txt | 167 |
1 files changed, 167 insertions, 0 deletions
diff --git a/Documentation/RCU/rculist_nulls.txt b/Documentation/RCU/rculist_nulls.txt new file mode 100644 index 00000000000..239f542d48b --- /dev/null +++ b/Documentation/RCU/rculist_nulls.txt @@ -0,0 +1,167 @@ +Using hlist_nulls to protect read-mostly linked lists and +objects using SLAB_DESTROY_BY_RCU allocations. + +Please read the basics in Documentation/RCU/listRCU.txt + +Using special makers (called 'nulls') is a convenient way +to solve following problem : + +A typical RCU linked list managing objects which are +allocated with SLAB_DESTROY_BY_RCU kmem_cache can +use following algos : + +1) Lookup algo +-------------- +rcu_read_lock() +begin: +obj = lockless_lookup(key); +if (obj) { + if (!try_get_ref(obj)) // might fail for free objects + goto begin; + /* + * Because a writer could delete object, and a writer could + * reuse these object before the RCU grace period, we + * must check key after geting the reference on object + */ + if (obj->key != key) { // not the object we expected + put_ref(obj); + goto begin; + } +} +rcu_read_unlock(); + +Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu() +but a version with an additional memory barrier (smp_rmb()) + +lockless_lookup(key) +{ + struct hlist_node *node, *next; + for (pos = rcu_dereference((head)->first); + pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && + ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); + pos = rcu_dereference(next)) + if (obj->key == key) + return obj; + return NULL; + +And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() : + + struct hlist_node *node; + for (pos = rcu_dereference((head)->first); + pos && ({ prefetch(pos->next); 1; }) && + ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); + pos = rcu_dereference(pos->next)) + if (obj->key == key) + return obj; + return NULL; +} + +Quoting Corey Minyard : + +"If the object is moved from one list to another list in-between the + time the hash is calculated and the next field is accessed, and the + object has moved to the end of a new list, the traversal will not + complete properly on the list it should have, since the object will + be on the end of the new list and there's not a way to tell it's on a + new list and restart the list traversal. I think that this can be + solved by pre-fetching the "next" field (with proper barriers) before + checking the key." + +2) Insert algo : +---------------- + +We need to make sure a reader cannot read the new 'obj->obj_next' value +and previous value of 'obj->key'. Or else, an item could be deleted +from a chain, and inserted into another chain. If new chain was empty +before the move, 'next' pointer is NULL, and lockless reader can +not detect it missed following items in original chain. + +/* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ +obj = kmem_cache_alloc(...); +lock_chain(); // typically a spin_lock() +obj->key = key; +atomic_inc(&obj->refcnt); +/* + * we need to make sure obj->key is updated before obj->next + */ +smp_wmb(); +hlist_add_head_rcu(&obj->obj_node, list); +unlock_chain(); // typically a spin_unlock() + + +3) Remove algo +-------------- +Nothing special here, we can use a standard RCU hlist deletion. +But thanks to SLAB_DESTROY_BY_RCU, beware a deleted object can be reused +very very fast (before the end of RCU grace period) + +if (put_last_reference_on(obj) { + lock_chain(); // typically a spin_lock() + hlist_del_init_rcu(&obj->obj_node); + unlock_chain(); // typically a spin_unlock() + kmem_cache_free(cachep, obj); +} + + + +-------------------------------------------------------------------------- +With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup() +and extra smp_wmb() in insert function. + +For example, if we choose to store the slot number as the 'nulls' +end-of-list marker for each slot of the hash table, we can detect +a race (some writer did a delete and/or a move of an object +to another chain) checking the final 'nulls' value if +the lookup met the end of chain. If final 'nulls' value +is not the slot number, then we must restart the lookup at +the begining. If the object was moved to same chain, +then the reader doesnt care : It might eventually +scan the list again without harm. + + +1) lookup algo + + head = &table[slot]; + rcu_read_lock(); +begin: + hlist_nulls_for_each_entry_rcu(obj, node, head, member) { + if (obj->key == key) { + if (!try_get_ref(obj)) // might fail for free objects + goto begin; + if (obj->key != key) { // not the object we expected + put_ref(obj); + goto begin; + } + goto out; + } +/* + * if the nulls value we got at the end of this lookup is + * not the expected one, we must restart lookup. + * We probably met an item that was moved to another chain. + */ + if (get_nulls_value(node) != slot) + goto begin; + obj = NULL; + +out: + rcu_read_unlock(); + +2) Insert function : +-------------------- + +/* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ +obj = kmem_cache_alloc(cachep); +lock_chain(); // typically a spin_lock() +obj->key = key; +atomic_set(&obj->refcnt, 1); +/* + * insert obj in RCU way (readers might be traversing chain) + */ +hlist_nulls_add_head_rcu(&obj->obj_node, list); +unlock_chain(); // typically a spin_unlock() |