Re: [RFC][PATCH] wake_up_var() memory ordering

From: David Howells <dhowells@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@lists.samba.org,
	Will Deacon <will.deacon@arm.com>,
	dri-devel@lists.freedesktop.org, 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,
	Andreas Gruenbacher <agruenba@redhat.com>,
	linux-rdma@vger.kernel.org, James Morris <jmorris@namei.org>,
	cluster-devel@redhat.com,
	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@hallyn.com>,
	Santosh Shilimkar <santosh.s>
Subject: Re: [RFC][PATCH] wake_up_var() memory ordering
Date: Tue, 25 Jun 2019 08:51:01 +0100	[thread overview]
Message-ID: <32379.1561449061@warthog.procyon.org.uk> (raw)
In-Reply-To: <20190624165012.GH3436@hirez.programming.kicks-ass.net>

Peter Zijlstra <peterz@infradead.org> wrote:

> I tried using wake_up_var() today and accidentally noticed that it
> didn't imply an smp_mb() and specifically requires it through
> wake_up_bit() / waitqueue_active().

Thinking about it again, I'm not sure why you need to add the barrier when
wake_up() (which this is a wrapper around) is required to impose a barrier at
the front if there's anything to wake up (ie. the wait queue isn't empty).

If this is insufficient, does it make sense just to have wake_up*() functions
do an unconditional release or full barrier right at the front, rather than it
being conditional on something being woken up?

> @@ -619,9 +614,7 @@ static int dvb_usb_fe_sleep(struct dvb_frontend *fe)
>  err:
>  	if (!adap->suspend_resume_active) {
>  		adap->active_fe = -1;

I'm wondering if there's a missing barrier here.  Should the clear_bit() on
the next line be clear_bit_unlock() or clear_bit_release()?

> -		clear_bit(ADAP_SLEEP, &adap->state_bits);
> -		smp_mb__after_atomic();
> -		wake_up_bit(&adap->state_bits, ADAP_SLEEP);
> +		clear_and_wake_up_bit(ADAP_SLEEP, &adap->state_bits);
>  	}
>  
>  	dev_dbg(&d->udev->dev, "%s: ret=%d\n", __func__, ret);
> diff --git a/fs/afs/fs_probe.c b/fs/afs/fs_probe.c
> index cfe62b154f68..377ee07d5f76 100644
> --- a/fs/afs/fs_probe.c
> +++ b/fs/afs/fs_probe.c
> @@ -18,6 +18,7 @@ static bool afs_fs_probe_done(struct afs_server *server)
>  
>  	wake_up_var(&server->probe_outstanding);
>  	clear_bit_unlock(AFS_SERVER_FL_PROBING, &server->flags);
> +	smp_mb__after_atomic();
>  	wake_up_bit(&server->flags, AFS_SERVER_FL_PROBING);
>  	return true;
>  }

Looking at this and the dvb one, does it make sense to stick the release
semantics of clear_bit_unlock() into clear_and_wake_up_bit()?

Also, should clear_bit_unlock() be renamed to clear_bit_release() (and
similarly test_and_set_bit_lock() -> test_and_set_bit_acquire()) if we seem to
be trying to standardise on that terminology.

David
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