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| author | Joel Fernandes (Google) <joel@joelfernandes.org> | 2018-10-09 03:33:41 +0200 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.ibm.com> | 2018-11-12 17:56:25 +0100 |
| commit | 93eb14201fc690687c2d94865bc38c1aa23356b8 (patch) | |
| tree | c724b95dbf03d099e65a96de629b7ea965e779db /Documentation/RCU | |
| parent | doc: rcu: Encourage use of rcu_barrier in checklist (diff) | |
| download | linux-93eb14201fc690687c2d94865bc38c1aa23356b8.tar.xz linux-93eb14201fc690687c2d94865bc38c1aa23356b8.zip | |
doc: Make reader aware of rcu_dereference_protected
The whatisRCU.txt document says rcu_dereference() cannot be used
outside of rcu_read_lock() protected sections. The commit adds a
mention of rcu_dereference_protected(), so that the new reader knows
that this API can be used to avoid update-side use of rcu_read_lock()
and rcu_read_unlock().
Cc: tytso@mit.edu
Suggested-by: tytso@mit.edu
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Update wording, including further feedback from Joel. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Diffstat (limited to 'Documentation/RCU')
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 7c33445fd0e5..4a6854318b17 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -266,7 +266,7 @@ rcu_dereference() unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid - only within the enclosing RCU read-side critical section. + only within the enclosing RCU read-side critical section [1]. For example, the following is -not- legal: rcu_read_lock(); @@ -292,6 +292,19 @@ rcu_dereference() typically used indirectly, via the _rcu list-manipulation primitives, such as list_for_each_entry_rcu(). + [1] The variant rcu_dereference_protected() can be used outside + of an RCU read-side critical section as long as the usage is + protected by locks acquired by the update-side code. This variant + avoids the lockdep warning that would happen when using (for + example) rcu_dereference() without rcu_read_lock() protection. + Using rcu_dereference_protected() also has the advantage + of permitting compiler optimizations that rcu_dereference() + must prohibit. The rcu_dereference_protected() variant takes + a lockdep expression to indicate which locks must be acquired + by the caller. If the indicated protection is not provided, + a lockdep splat is emitted. See RCU/Design/Requirements.html + and the API's code comments for more details and example usage. + The following diagram shows how each API communicates among the reader, updater, and reclaimer. |