Commit-ID: 5726ce06ad6bcd8dd75a204d1465c99a2f897d3a Gitweb: http://git.kernel.org/tip/5726ce06ad6bcd8dd75a204d1465c99a2f897d3a Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Tue, 13 May 2014 10:14:51 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Tue, 8 Jul 2014 08:13:03 -0700 documentation: Clarify wake-up/memory-barrier relationship This commit adds an example demonstrating that if a wake_up() doesn't actually wake something up, no memory ordering is provided. Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> Reviewed-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx> Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> --- Documentation/memory-barriers.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f1dc4a2..a6ca533 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1893,6 +1893,21 @@ between the STORE to indicate the event and the STORE to set TASK_RUNNING: <general barrier> STORE current->state LOAD event_indicated +To repeat, this write memory barrier is present if and only if something +is actually awakened. To see this, consider the following sequence of +events, where X and Y are both initially zero: + + CPU 1 CPU 2 + =============================== =============================== + X = 1; STORE event_indicated + smp_mb(); wake_up(); + Y = 1; wait_event(wq, Y == 1); + wake_up(); load from Y sees 1, no memory barrier + load from X might see 0 + +In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed +to see 1. + The available waker functions include: complete(); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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