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Message-Id: <1490045915-18759-1-git-send-email-mixaskok@gmail.com> Date: Mon, 20 Mar 2017 22:38:35 +0100 From: Michalis Kokologiannakis <mixaskok@...il.com> To: paulmck@...ux.vnet.ibm.com Cc: linux-kernel@...r.kernel.org Subject: [PATCH] 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@...il.com> --- 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 -- 2.1.4
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