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    Subject[PATCH RFC 04/26] completion: Replace spin_unlock_wait() with lock/unlock pair
    Date
    There is no agreed-upon definition of spin_unlock_wait()'s semantics,
    and it appears that all callers could do just as well with a lock/unlock
    pair. This commit therefore replaces the spin_unlock_wait() call in
    completion_done() with spin_lock() followed immediately by spin_unlock().
    This should be safe from a performance perspective because the lock
    will be held only the wakeup happens really quickly.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: Alan Stern <stern@rowland.harvard.edu>
    Cc: Andrea Parri <parri.andrea@gmail.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    ---
    kernel/sched/completion.c | 9 ++-------
    1 file changed, 2 insertions(+), 7 deletions(-)

    diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
    index 53f9558fa925..d4b89a59629f 100644
    --- a/kernel/sched/completion.c
    +++ b/kernel/sched/completion.c
    @@ -307,14 +307,9 @@ bool completion_done(struct completion *x)
    * If ->done, we need to wait for complete() to release ->wait.lock
    * otherwise we can end up freeing the completion before complete()
    * is done referencing it.
    - *
    - * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
    - * the loads of ->done and ->wait.lock such that we cannot observe
    - * the lock before complete() acquires it while observing the ->done
    - * after it's acquired the lock.
    */
    - smp_rmb();
    - spin_unlock_wait(&x->wait.lock);
    + spin_lock(&x->wait.lock);
    + spin_unlock(&x->wait.lock);
    return true;
    }
    EXPORT_SYMBOL(completion_done);
    --
    2.5.2
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