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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@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
hifive-unleashed-5.1
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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9
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
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