Commit dec13c42 authored Jun 29, 2017 by Paul E. McKenney

completion: Replace spin_unlock_wait() with lock/unlock pair



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>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

parent 23a9b748

kernel/sched/completion.c

+4 −7

Original line number	Diff line number	Diff line
		@@ -300,6 +300,8 @@ EXPORT_SYMBOL(try_wait_for_completion);
		*/
		bool completion_done(struct completion *x)
		{
		unsigned long flags;

		if (!READ_ONCE(x->done))
		return false;

		@@ -307,14 +309,9 @@ 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_irqsave(&x->wait.lock, flags);
		spin_unlock_irqrestore(&x->wait.lock, flags);
		return true;
		}
		EXPORT_SYMBOL(completion_done);