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| author | Michalis Kokologiannakis <mixaskok@gmail.com> | 2017-03-20 22:38:35 +0100 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2017-04-12 08:23:42 -0700 |
| commit | 93728af0a1f63e13d6f7f56a434965b05b8b2abd (patch) | |
| tree | 51e775a3be39b18e623c5726eece18b7bcc5a92c | |
| parent | 066bb1c84aa430d15f36070471cbfe8976631cce (diff) | |
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>
| -rw-r--r-- | Documentation/RCU/rcu_dereference.txt | 9 |
1 files changed, 9 insertions, 0 deletions
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 |