Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair

Paul E. McKenney
2017-08-17T15:07:47Z

On Wed, Aug 16, 2017 at 11:22:35AM -0400, Steven Rostedt wrote:
> On Tue, 15 Aug 2017 09:16:29 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > 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>
> > [ paulmck: Updated to use irqsave based on 0day Test Robot feedback. ]
> >
> > diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> > index 13fc5ae9bf2f..c9524d2d9316 100644
> > --- a/kernel/sched/completion.c
> > +++ b/kernel/sched/completion.c
> > @@ -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);
>
> For this patch:
>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Applied, thank you!

> But I was looking at this function, and it is a little worrisome, as it
> says it should return false if there are waiters and true otherwise.
> But it can also return false if there are no waiters and the completion
> is already done.
>
> Basically we have:
>
> wait_for_completion() {
> while (!done)
> wait();
> done--;
> }
>
> complete() {
> done++;
> wake_up_waiters();
> }
>
> Thus, completion_done() only returns true if a complete happened and a
> wait_for_completion has not. It does not return true if the complete
> has not yet occurred, but there are still waiters.
>
> I looked at a couple of use cases, and this does not appear to be an
> issue, but the documentation about the completion_done() does not
> exactly fit the implementation. Should that be addressed?
>
> Also, if complete_all() is called, then reinit_completion() must be
> called before that completion is used. The reinit_completion() has a
> comment stating this, but there's no comment by complete_all() stating
> this, which is where it really should be. I'll send a patch to fix this
> one.

But I am too late to return the favor -- good patch, though!

Thanx, Paul