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Subject[PATCH tip/core/rcu 7/9] doc: Update the comparisons rule in rcu_dereference.txt
Date
From: Michalis Kokologiannakis <mixaskok@gmail.com>

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@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.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 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
--
2.5.2
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