Messages in this thread | | | Date | Thu, 17 Aug 2017 10:26:16 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair |
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* 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);
I'm fine with this patch - as long as there are no performance regression reports. (which I suspect there won't be.)
Would you like to carry this in the RCU tree, due to other changes depending on this change - or can I pick this up into the scheduler tree?
Thanks,
Ingo
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