From: Michalis Kokologiannakis Date: Mon, 20 Mar 2017 21:38:35 +0000 (+0100) Subject: doc: Update the comparisons rule in rcu_dereference.txt X-Git-Tag: v4.12-rc1~38^2~5^2~3 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=93728af0a1f63e13d6f7f56a434965b05b8b2abd;p=karo-tx-linux.git doc: Update the comparisons rule in rcu_dereference.txt When an RCU-protected pointer is fetched but never dereferenced rcu_access_pointer() should be used in place of rcu_dereference(). This commit explicitly records this very fact in Documentation/ RCU/rcu_dereference.txt, in order to prevent the usage of rcu_dereference() in comparisons. Signed-off-by: Michalis Kokologiannakis Signed-off-by: Paul E. McKenney --- diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt index c0bf2441a2ba..b2a613f16d74 100644 --- a/Documentation/RCU/rcu_dereference.txt +++ b/Documentation/RCU/rcu_dereference.txt @@ -138,6 +138,15 @@ o Be very careful about comparing pointers obtained from This sort of comparison occurs frequently when scanning RCU-protected circular linked lists. + Note that if checks for being within an RCU read-side + critical section are not required and the pointer is never + dereferenced, rcu_access_pointer() should be used in place + of rcu_dereference(). The rcu_access_pointer() primitive + does not require an enclosing read-side critical section, + and also omits the smp_read_barrier_depends() included in + rcu_dereference(), which in turn should provide a small + performance gain in some CPUs (e.g., the DEC Alpha). + o The comparison is against a pointer that references memory that was initialized "a long time ago." The reason this is safe is that even if misordering occurs, the