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doc: Update the comparisons rule in rcu_dereference.txt

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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>
This commit is contained in:
Michalis Kokologiannakis 2017-03-20 22:38:35 +01:00 committed by Paul E. McKenney
parent 066bb1c84a
commit 93728af0a1
1 changed files with 9 additions and 0 deletions
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Documentation/RCU/rcu_dereference.txt
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@ -138,6 +138,15 @@ o Be very careful about comparing pointers obtained from
This sort of comparison occurs frequently when scanning This sort of comparison occurs frequently when scanning
RCU-protected circular linked lists. 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 o The comparison is against a pointer that references memory
that was initialized "a long time ago." The reason that was initialized "a long time ago." The reason
this is safe is that even if misordering occurs, the this is safe is that even if misordering occurs, the