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Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
Date 2016-11-25 18:50 +0100
Message-ID <sHsNk-3Ai-7@gated-at.bofh.it> (permalink)
References (5 earlier) <sHqBQ-2hw-29@gated-at.bofh.it> <sHroe-2Rp-5@gated-at.bofh.it> <sHroe-2Rp-7@gated-at.bofh.it> <sHs0W-34x-17@gated-at.bofh.it> <sHstY-3tw-29@gated-at.bofh.it>
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On Fri, Nov 25, 2016 at 05:28:01PM +0000, Mark Rutland wrote:
> On Fri, Nov 25, 2016 at 05:49:45PM +0100, Christian Borntraeger wrote:
> > On 11/25/2016 05:17 PM, Peter Zijlstra wrote:

> > There were several cases that I found during writing the *ONCE stuff.
> > For example there are some 32bit ppc variants with 64bit PTEs. Some for
> > others (I think sparc).
> 
> We have similar on 32-bit ARM w/ LPAE. LPAE implies that a naturally
> aligned 64-bit access is single-copy atomic, which is what makes that
> ok.
> 
> > And the mm/ code is perfectly fine with these PTE accesses being done
> > NOT atomic.
> 
> That strikes me as surprising. Is there some mutual exclusion that
> prevents writes from occuring wherever a READ_ONCE() happens to a PTE?
> 
> Otherwise, how is tearing not a problem? Does it have some pattern like
> the lockref cmpxchg?

On x86 PAE we play silly games, see arch/x86/mm/gup.c:gup_get_ptr().

Those two loads really should be READ_ONCE()/LOAD_SINGLE().

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[PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-24 11:30 +0100
  [PATCH 2/3] vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-24 11:30 +0100
    Re: [PATCH 2/3] vringh: kill off ACCESS_ONCE() Christian Borntraeger <borntraeger@de.ibm.com> - 2016-11-24 12:20 +0100
    Re: [PATCH 2/3] vringh: kill off ACCESS_ONCE() Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-11-24 12:40 +0100
    Re: [PATCH 2/3] vringh: kill off ACCESS_ONCE() Jason Wang <jasowang@redhat.com> - 2016-11-25 03:50 +0100
  [PATCH 1/3] tools/virtio: fix READ_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-24 11:30 +0100
    Re: [PATCH 1/3] tools/virtio: fix READ_ONCE() Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-11-24 12:40 +0100
    Re: [PATCH 1/3] tools/virtio: fix READ_ONCE() Jason Wang <jasowang@redhat.com> - 2016-11-25 03:40 +0100
  [PATCH 3/3] tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h Mark Rutland <mark.rutland@arm.com> - 2016-11-24 11:30 +0100
    Re: [PATCH 3/3] tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h Cornelia Huck <cornelia.huck@de.ibm.com> - 2016-11-24 12:40 +0100
    Re: [PATCH 3/3] tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h Jason Wang <jasowang@redhat.com> - 2016-11-25 03:50 +0100
  Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() "Michael S. Tsirkin" <mst@redhat.com> - 2016-11-24 21:40 +0100
    Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 12:30 +0100
      Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Christian Borntraeger <borntraeger@de.ibm.com> - 2016-11-25 12:40 +0100
        Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 13:30 +0100
          Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Peter Zijlstra <peterz@infradead.org> - 2016-11-25 13:50 +0100
            Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Peter Zijlstra <peterz@infradead.org> - 2016-11-25 13:50 +0100
              Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Boqun Feng <boqun.feng@gmail.com> - 2016-11-25 16:00 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Dmitry Vyukov <dvyukov@google.com> - 2016-11-25 16:30 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 17:20 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Peter Zijlstra <peterz@infradead.org> - 2016-11-25 17:20 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 17:40 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Christian Borntraeger <borntraeger@de.ibm.com> - 2016-11-25 18:00 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 18:30 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Peter Zijlstra <peterz@infradead.org> - 2016-11-25 18:50 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Christian Borntraeger <borntraeger@de.ibm.com> - 2016-11-25 19:50 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() "Michael S. Tsirkin" <mst@redhat.com> - 2016-11-25 22:10 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Christian Borntraeger <borntraeger@de.ibm.com> - 2016-11-25 22:50 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Dmitry Vyukov <dvyukov@google.com> - 2016-11-25 18:30 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 18:50 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Linus Torvalds <torvalds@linux-foundation.org> - 2016-11-25 19:00 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 19:10 +0100
                Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Linus Torvalds <torvalds@linux-foundation.org> - 2016-11-25 19:50 +0100
            Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE() Mark Rutland <mark.rutland@arm.com> - 2016-11-25 15:40 +0100

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