forked from luck/tmp_suning_uos_patched
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>
This commit is contained in:
parent
1c7d6d4411
commit
93eb14201f
|
@ -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.
|
||||
|
||||
|
|
Loading…
Reference in New Issue
Block a user