Commit-ID: 93eb14201fc690687c2d94865bc38c1aa23356b8 Gitweb: https://git.kernel.org/tip/93eb14201fc690687c2d94865bc38c1aa23356b8 Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> AuthorDate: Mon, 8 Oct 2018 18:33:41 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> CommitDate: Mon, 12 Nov 2018 08:56:25 -0800 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@xxxxxxx Suggested-by: tytso@xxxxxxx Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> [ paulmck: Update wording, including further feedback from Joel. ] Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> --- Documentation/RCU/whatisRCU.txt | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) 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.
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