Commit-ID: 93728af0a1f63e13d6f7f56a434965b05b8b2abd Gitweb: http://git.kernel.org/tip/93728af0a1f63e13d6f7f56a434965b05b8b2abd Author: Michalis Kokologiannakis <mixaskok@xxxxxxxxx> AuthorDate: Mon, 20 Mar 2017 22:38:35 +0100 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Wed, 12 Apr 2017 08:23:42 -0700 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 <mixaskok@xxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- Documentation/RCU/rcu_dereference.txt | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt index c0bf244..b2a613f 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 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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