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From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	dbueso@suse.de, jasowang@redhat.com,
	KVM list <kvm@vger.kernel.org>, netdev <netdev@vger.kernel.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	virtualization@lists.linux-foundation.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
Date: Fri, 25 Nov 2016 16:32:21 +0000	[thread overview]
Message-ID: <20161125163220.GB30181@leverpostej> (raw)
In-Reply-To: <20161125161709.GQ3092@twins.programming.kicks-ass.net>

On Fri, Nov 25, 2016 at 05:17:09PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 25, 2016 at 04:10:04PM +0000, Mark Rutland wrote:
> > On Fri, Nov 25, 2016 at 04:21:39PM +0100, Dmitry Vyukov wrote:
> 
> > > What are use cases for such primitive that won't be OK with "read once
> > > _and_ atomically"?
> > 
> > I have none to hand.
> 
> Whatever triggers the __builtin_memcpy() paths, and even the size==8
> paths on 32bit.

Lockref, per:

http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02294.html

In that specific case, a torn value just means we'll retry until we get
a non torn value, due to the cmpxchg. For that case, all we need is the
value to be reloaded per invocation of READ_ONCE().

This guy seems to have the full story:

http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02389.html
http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02558.html

Thanks,
Mark.

  reply	other threads:[~2016-11-25 16:33 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-24 10:25 [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland
2016-11-24 10:25 ` [PATCH 1/3] tools/virtio: fix READ_ONCE() Mark Rutland
2016-11-24 11:34   ` Cornelia Huck
2016-11-25  2:38   ` Jason Wang
2016-11-24 10:25 ` [PATCH 2/3] vringh: kill off ACCESS_ONCE() Mark Rutland
2016-11-24 11:10   ` Christian Borntraeger
2016-11-24 11:35   ` Cornelia Huck
2016-11-25  2:40   ` Jason Wang
2016-11-24 10:25 ` [PATCH 3/3] tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h Mark Rutland
2016-11-24 11:37   ` Cornelia Huck
2016-11-25  2:40   ` Jason Wang
2016-11-24 20:36 ` [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Michael S. Tsirkin
2016-11-25 11:22   ` Mark Rutland
2016-11-25 11:33     ` Christian Borntraeger
2016-11-25 12:23       ` Mark Rutland
2016-11-25 12:40         ` Peter Zijlstra
2016-11-25 12:44           ` Peter Zijlstra
2016-11-25 14:56             ` Boqun Feng
2016-11-25 15:21               ` Dmitry Vyukov
2016-11-25 16:10                 ` Mark Rutland
2016-11-25 16:17                   ` Peter Zijlstra
2016-11-25 16:32                     ` Mark Rutland [this message]
2016-11-25 16:49                     ` Christian Borntraeger
2016-11-25 17:28                       ` Mark Rutland
2016-11-25 17:42                         ` Peter Zijlstra
2016-11-25 18:46                         ` Christian Borntraeger
2016-11-25 21:08                       ` Michael S. Tsirkin
2016-11-25 21:45                         ` Christian Borntraeger
2016-11-25 17:28                     ` Dmitry Vyukov
2016-11-25 17:43                       ` Mark Rutland
2016-11-25 17:52                       ` Linus Torvalds
2016-11-25 18:07                         ` Mark Rutland
2016-11-25 18:47                           ` Linus Torvalds
2016-11-25 14:35           ` Mark Rutland

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