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From: Andreas Gruenbacher <agruenba@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Martin Brandenburg <martin@omnibond.com>,
	linux-cachefs@redhat.com, Mike Snitzer <snitzer@redhat.com>,
	linux-aio@kvack.org, David Airlie <airlied@linux.ie>,
	samba-technical <samba-technical@lists.samba.org>,
	Will Deacon <will.deacon@arm.com>,
	dri-devel@lists.freedesktop.org,
	David Howells <dhowells@redhat.com>, Chris Mason <clm@fb.com>,
	dm-devel@redhat.com, keyrings@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	linux-afs@lists.infradead.org, Alasdair Kergon <agk@redhat.com>,
	Mike Marshall <hubcap@omnibond.com>,
	linux-cifs@vger.kernel.org, rds-devel@oss.oracle.com,
	linux-rdma@vger.kernel.org, James Morris <jmorris@namei.org>,
	cluster-devel <cluster-devel@redhat.com>,
	Antti Palosaari <crope@iki.fi>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	intel-gfx@lists.freedesktop.org, devel@lists.orangefs.org,
	"Serge E. Hallyn" <serge@ha>
Subject: Re: [RFC][PATCH] wake_up_var() memory ordering
Date: Tue, 25 Jun 2019 14:12:22 +0200	[thread overview]
Message-ID: <CAHc6FU6zUCdQZ1AfN2KYcPYVKc5bwvc0bD7=-KZpFXws+F9QZQ@mail.gmail.com> (raw)
In-Reply-To: <20190625103430.GW3402@hirez.programming.kicks-ass.net>

On Tue, 25 Jun 2019 at 12:36, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Jun 25, 2019 at 11:19:35AM +0200, Andreas Gruenbacher wrote:
> > > diff --git a/fs/gfs2/glops.c b/fs/gfs2/glops.c
> > > index cf4c767005b1..666629ea5da7 100644
> > > --- a/fs/gfs2/glops.c
> > > +++ b/fs/gfs2/glops.c
> > > @@ -227,6 +227,7 @@ static void gfs2_clear_glop_pending(struct gfs2_inode *ip)
> > >                 return;
> > >
> > >         clear_bit_unlock(GIF_GLOP_PENDING, &ip->i_flags);
> > > +       smp_mb__after_atomic();
> > >         wake_up_bit(&ip->i_flags, GIF_GLOP_PENDING);
> >
> > This should become clear_and_wake_up_bit as well, right? There are
> > several more instances of the same pattern.
>
> Only if we do as David suggested and make clean_and_wake_up_bit()
> provide the RELEASE barrier.

(It's clear_and_wake_up_bit, not clean_and_wake_up_bit.)

> That is, currently clear_and_wake_up_bit() is
>
>         clear_bit()
>         smp_mb__after_atomic();
>         wake_up_bit();
>
> But the above is:
>
>         clear_bit_unlock();
>         smp_mb__after_atomic();
>         wake_up_bit()
>
> the difference is that _unlock() uses RELEASE semantics, where
> clear_bit() does not.
>
> The difference is illustrated with something like:
>
>         cond = true;
>         clear_bit()
>         smp_mb__after_atomic();
>         wake_up_bit();
>
> In this case, a remote CPU can first observe the clear_bit() and then
> the 'cond = true' store. When we use clear_bit_unlock() this is not
> possible, because the RELEASE barrier ensures that everything before,
> stays before.

Now I'm confused because clear_and_wake_up_bit() in mainline does use
clear_bit_unlock(), so it's the exact opposite of what you just said.

Thanks,
Andreas
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx

  reply	other threads:[~2019-06-25 12:12 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-24 16:50 Peter Zijlstra
2019-06-25  7:51 ` David Howells
     [not found]   ` <32379.1561449061-S6HVgzuS8uM4Awkfq6JHfwNdhmdF6hFW@public.gmane.org>
2019-06-25  8:11     ` Peter Zijlstra
2019-06-25  9:19 ` Andreas Gruenbacher
     [not found]   ` <CAHc6FU7j5iW7WQoxN_OSfvK4zxv_MxTWJpiNsqFW8TEDMX1rjw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-06-25 10:34     ` Peter Zijlstra
2019-06-25 12:12       ` Andreas Gruenbacher [this message]
     [not found]         ` <CAHc6FU6zUCdQZ1AfN2KYcPYVKc5bwvc0bD7=-KZpFXws+F9QZQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-06-25 13:27           ` Peter Zijlstra

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