diff options
Diffstat (limited to 'Documentation/RCU/listRCU.txt')
| -rw-r--r-- | Documentation/RCU/listRCU.txt | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.txt index bda6ead69bd..f8a54fa0d8a 100644 --- a/Documentation/RCU/listRCU.txt +++ b/Documentation/RCU/listRCU.txt @@ -32,6 +32,7 @@ implementation of audit_filter_task() might be as follows: enum audit_state state; read_lock(&auditsc_lock); + /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { read_unlock(&auditsc_lock); @@ -55,6 +56,7 @@ This means that RCU can be easily applied to the read side, as follows: enum audit_state state; rcu_read_lock(); + /* Note: audit_netlink_sem held by caller. */ list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { rcu_read_unlock(); @@ -139,12 +141,15 @@ Normally, the write_lock() and write_unlock() would be replaced by a spin_lock() and a spin_unlock(), but in this case, all callers hold audit_netlink_sem, so no additional locking is required. The auditsc_lock can therefore be eliminated, since use of RCU eliminates the need for -writers to exclude readers. +writers to exclude readers. Normally, the write_lock() calls would +be converted into spin_lock() calls. The list_del(), list_add(), and list_add_tail() primitives have been replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). The _rcu() list-manipulation primitives add memory barriers that are -needed on weakly ordered CPUs (most of them!). +needed on weakly ordered CPUs (most of them!). The list_del_rcu() +primitive omits the pointer poisoning debug-assist code that would +otherwise cause concurrent readers to fail spectacularly. So, when readers can tolerate stale data and when entries are either added or deleted, without in-place modification, it is very easy to use RCU! @@ -166,6 +171,7 @@ otherwise, the added fields would need to be filled in): struct audit_newentry *ne; write_lock(&auditsc_lock); + /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { e->rule.action = newaction; @@ -199,8 +205,7 @@ RCU ("read-copy update") its name. The RCU code is as follows: audit_copy_rule(&ne->rule, &e->rule); ne->rule.action = newaction; ne->rule.file_count = newfield_count; - list_add_rcu(ne, e); - list_del(e); + list_replace_rcu(e, ne); call_rcu(&e->rcu, audit_free_rule, e); return 0; } |