Messages in this thread | | | Date | Thu, 17 Aug 2017 08:07:36 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair |
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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
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