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From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
To: Andreas Gruenbacher <agruenba-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Martin Brandenburg
	<martin-gqc3UtWaqJ5Wk0Htik3J/w@public.gmane.org>,
	linux-cachefs-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org,
	Mike Snitzer <snitzer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	linux-aio-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
	David Airlie <airlied-cv59FeDIM0c@public.gmane.org>,
	samba-technical
	<samba-technical-w/Ol4Ecudpl8XjKLYN78aQ@public.gmane.org>,
	Joonas Lahtinen
	<joonas.lahtinen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>,
	Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>,
	dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org,
	David Howells <dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Chris Mason <clm-b10kYP2dOMg@public.gmane.org>,
	dm-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org,
	keyrings-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	linux-afs-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org,
	Alasdair Kergon <agk-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Mike Marshall <hubcap-gqc3UtWaqJ5Wk0Htik3J/w@public.gmane.org>,
	linux-cifs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	rds-devel-N0ozoZBvEnrZJqsBc5GL+g@public.gmane.org,
	linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	James Morris <jmorris-gx6/JNMH7DfYtjvyW6yDsg@public.gmane.org>,
	cluster-devel
	<cluster-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Antti Palosaari <crope-X3B1VOXEql0@public.gmane.org>,
	Matthias Brugger
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	Paul McKenney
	<paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
	intel-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.orgd
Subject: Re: [RFC][PATCH] wake_up_var() memory ordering
Date: Tue, 25 Jun 2019 12:34:30 +0200	[thread overview]
Message-ID: <20190625103430.GW3402@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHc6FU7j5iW7WQoxN_OSfvK4zxv_MxTWJpiNsqFW8TEDMX1rjw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

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.

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.

> >  }
> >

  parent reply	other threads:[~2019-06-25 10:34 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 [this message]
2019-06-25 12:12       ` Andreas Gruenbacher
     [not found]         ` <CAHc6FU6zUCdQZ1AfN2KYcPYVKc5bwvc0bD7=-KZpFXws+F9QZQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-06-25 13:27           ` Peter Zijlstra

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