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 <matthias.bgg-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, 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. > > } > >
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org> To: cluster-devel.redhat.com Subject: [Cluster-devel] [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@mail.gmail.com> 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. > > } > >
next prev parent reply other threads:[~2019-06-25 10:34 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-06-24 16:50 [RFC][PATCH] wake_up_var() memory ordering Peter Zijlstra 2019-06-24 16:50 ` [Cluster-devel] " Peter Zijlstra 2019-06-25 7:51 ` David Howells 2019-06-25 7:51 ` [Cluster-devel] " David Howells [not found] ` <32379.1561449061-S6HVgzuS8uM4Awkfq6JHfwNdhmdF6hFW@public.gmane.org> 2019-06-25 8:11 ` Peter Zijlstra 2019-06-25 8:11 ` [Cluster-devel] " Peter Zijlstra 2019-06-25 9:19 ` Andreas Gruenbacher 2019-06-25 9:19 ` [Cluster-devel] " Andreas Gruenbacher [not found] ` <CAHc6FU7j5iW7WQoxN_OSfvK4zxv_MxTWJpiNsqFW8TEDMX1rjw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org> 2019-06-25 10:34 ` Peter Zijlstra [this message] 2019-06-25 10:34 ` Peter Zijlstra 2019-06-25 12:12 ` Andreas Gruenbacher 2019-06-25 12:12 ` [Cluster-devel] " Andreas Gruenbacher [not found] ` <CAHc6FU6zUCdQZ1AfN2KYcPYVKc5bwvc0bD7=-KZpFXws+F9QZQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org> 2019-06-25 13:27 ` Peter Zijlstra 2019-06-25 13:27 ` [Cluster-devel] " Peter Zijlstra
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