From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
linux-arch@vger.kernel.org,
Andrew Cooper <andrew.cooper3@citrix.com>,
Russell King - ARM Linux <linux@arm.linux.org.uk>,
virtualization@lists.linux-foundation.org,
Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
"H. Peter Anvin" <hpa@zytor.com>, Joe Perches <joe@perches.com>,
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Ralf Baechle <ralf@linux-mips.org>,
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Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h
Date: Tue, 26 Jan 2016 11:58:20 -0800 [thread overview]
Message-ID: <20160126195820.GS4503@linux.vnet.ibm.com> (raw)
In-Reply-To: <20160126121608.GE21553@arm.com>
On Tue, Jan 26, 2016 at 12:16:09PM +0000, Will Deacon wrote:
> On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote:
> > > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote:
> > > > PPC Overlapping Group-B sets version 4
> > > > ""
> > > > (* When the Group-B sets from two different barriers involve instructions in
> > > > the same thread, within that thread one set must contain the other.
> > > >
> > > > P0 P1 P2
> > > > Rx=1 Wy=1 Wz=2
> > > > dep. lwsync lwsync
> > > > Ry=0 Wz=1 Wx=1
> > > > Rz=1
> > > >
> > > > assert(!(z=2))
> > > >
> > > > Forbidden by ppcmem, allowed by herd.
> > > > *)
> > > > {
> > > > 0:r1=x; 0:r2=y; 0:r3=z;
> > > > 1:r1=x; 1:r2=y; 1:r3=z; 1:r4=1;
> > > > 2:r1=x; 2:r2=y; 2:r3=z; 2:r4=1; 2:r5=2;
> > > > }
> > > > P0 | P1 | P2 ;
> > > > lwz r6,0(r1) | stw r4,0(r2) | stw r5,0(r3) ;
> > > > xor r7,r6,r6 | lwsync | lwsync ;
> > > > lwzx r7,r7,r2 | stw r4,0(r3) | stw r4,0(r1) ;
> > > > lwz r8,0(r3) | | ;
> > > >
> > > > exists
> > > > (z=2 /\ 0:r6=1 /\ 0:r7=0 /\ 0:r8=1)
> > >
> > > That really hurts. Assuming that the "assert(!(z=2))" is actually there
> > > to constrain the coherence order of z to be {0->1->2}, then I think that
> > > this test is forbidden on arm using dmb instead of lwsync. That said, I
> > > also don't think the Rz=1 in P0 changes anything.
> >
> > What about the smp_wmb() variant of dmb that orders only stores?
>
> Tricky, but I think it still works out if the coherence order of z is as
> I described above. The line of reasoning is weird though -- I ended up
> considering the two cases where P0 reads z before and after it reads x
> and what that means for the read of y.
By "works out" you mean that ARM prohibits the outcome?
BTW, I never have seen a real-world use for this case. At the moment
it is mostly a cautionary tale about memory-model corner cases and
tools.
Thanx, Paul
next prev parent reply other threads:[~2016-01-26 21:59 UTC|newest]
Thread overview: 155+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-10 14:16 [PATCH v3 00/41] arch: barrier cleanup + barriers for virt Michael S. Tsirkin
2016-01-10 14:16 ` [PATCH v3 01/41] lcoking/barriers, arch: Use smp barriers in smp_store_release() Michael S. Tsirkin
2016-01-12 16:28 ` Paul E. McKenney
2016-01-12 18:40 ` Michael S. Tsirkin
2016-01-10 14:16 ` [PATCH v3 02/41] asm-generic: guard smp_store_release/load_acquire Michael S. Tsirkin
2016-01-10 14:16 ` [PATCH v3 03/41] ia64: rename nop->iosapic_nop Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 04/41] ia64: reuse asm-generic/barrier.h Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 05/41] powerpc: " Michael S. Tsirkin
2016-01-12 16:31 ` Paul E. McKenney
2016-01-10 14:17 ` [PATCH v3 06/41] s390: " Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 07/41] sparc: " Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 08/41] arm: " Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 09/41] arm64: " Michael S. Tsirkin
2016-01-10 14:17 ` [PATCH v3 10/41] metag: " Michael S. Tsirkin
2016-01-10 14:18 ` [PATCH v3 11/41] mips: " Michael S. Tsirkin
2016-01-12 1:14 ` [v3,11/41] " Leonid Yegoshin
2016-01-12 8:43 ` Michael S. Tsirkin
2016-01-12 9:51 ` Peter Zijlstra
2016-01-12 9:27 ` Peter Zijlstra
2016-01-12 10:25 ` Peter Zijlstra
2016-01-12 10:40 ` Peter Zijlstra
2016-01-12 11:41 ` Will Deacon
2016-01-12 20:45 ` Leonid Yegoshin
2016-01-12 21:40 ` Peter Zijlstra
2016-01-13 0:21 ` Leonid Yegoshin
2016-01-13 10:45 ` Will Deacon
2016-01-13 19:02 ` Leonid Yegoshin
2016-01-13 20:48 ` Peter Zijlstra
2016-01-13 20:58 ` Leonid Yegoshin
2016-01-14 12:04 ` Will Deacon
2016-01-14 16:16 ` Paul E. McKenney
2016-01-14 19:42 ` Leonid Yegoshin
2016-01-14 20:15 ` Peter Zijlstra
2016-01-14 20:36 ` Paul E. McKenney
2016-01-14 20:46 ` Peter Zijlstra
2016-01-14 20:46 ` Leonid Yegoshin
2016-01-14 21:34 ` Paul E. McKenney
2016-01-14 21:45 ` Leonid Yegoshin
2016-01-14 22:24 ` Paul E. McKenney
2016-01-14 23:04 ` Leonid Yegoshin
2016-01-14 20:12 ` Leonid Yegoshin
2016-01-14 20:48 ` Paul E. McKenney
2016-01-14 21:24 ` Leonid Yegoshin
2016-01-14 22:20 ` Paul E. McKenney
2016-01-15 9:57 ` Will Deacon
2016-01-15 18:54 ` Leonid Yegoshin
2016-01-26 10:24 ` Peter Zijlstra
2016-01-26 10:32 ` Peter Zijlstra
2016-01-26 11:09 ` Will Deacon
2016-01-26 20:11 ` Paul E. McKenney
2016-01-27 8:35 ` [PATCH] documentation: Add disclaimer Peter Zijlstra
2016-01-27 10:11 ` Will Deacon
2016-04-14 21:40 ` Paul E. McKenney
2016-01-27 14:57 ` David Howells
2016-01-27 23:35 ` Paul E. McKenney
2016-01-28 20:02 ` David Howells
2016-04-14 21:40 ` Paul E. McKenney
2016-01-26 19:44 ` [v3,11/41] mips: reuse asm-generic/barrier.h Paul E. McKenney
2016-01-18 8:19 ` Herbert Xu
2016-01-18 15:46 ` Paul E. McKenney
2016-01-26 16:52 ` Boqun Feng
2016-01-26 17:22 ` Peter Zijlstra
2016-01-26 19:44 ` Linus Torvalds
2016-01-26 20:10 ` Paul E. McKenney
2016-01-26 22:15 ` Linus Torvalds
2016-01-26 22:33 ` Linus Torvalds
2016-01-26 23:29 ` Paul E. McKenney
2016-01-26 23:45 ` Linus Torvalds
2016-01-27 0:57 ` Paul E. McKenney
2016-01-27 2:04 ` Boqun Feng
2016-01-27 23:30 ` Paul E. McKenney
2016-01-27 7:51 ` Peter Zijlstra
2016-01-27 8:05 ` Linus Torvalds
2016-01-26 19:51 ` Paul E. McKenney
2016-01-13 22:26 ` Leonid Yegoshin
2016-01-14 9:24 ` Michael S. Tsirkin
2016-01-14 12:14 ` Will Deacon
2016-01-14 19:28 ` Leonid Yegoshin
2016-01-14 20:34 ` Paul E. McKenney
2016-01-14 21:01 ` Leonid Yegoshin
2016-01-14 21:29 ` Paul E. McKenney
2016-01-14 21:36 ` Leonid Yegoshin
2016-01-14 22:55 ` Paul E. McKenney
2016-01-14 23:33 ` Leonid Yegoshin
2016-01-15 0:47 ` Paul E. McKenney
2016-01-15 1:07 ` Leonid Yegoshin
2016-01-27 11:26 ` Maciej W. Rozycki
2016-01-28 0:48 ` Leonid Yegoshin
2016-01-29 13:38 ` Maciej W. Rozycki
2016-01-28 0:58 ` Leonid Yegoshin
2016-01-27 10:40 ` Ralf Baechle
2016-01-27 12:09 ` Maciej W. Rozycki
2016-01-15 10:24 ` Will Deacon
2016-01-15 17:54 ` Paul E. McKenney
2016-01-15 19:28 ` Paul E. McKenney
2016-01-25 14:41 ` Will Deacon
2016-01-26 1:06 ` Paul E. McKenney
2016-01-26 12:10 ` Will Deacon
2016-01-26 23:37 ` Paul E. McKenney
2016-01-27 10:23 ` Will Deacon
2016-01-15 8:55 ` Peter Zijlstra
2016-01-15 9:13 ` Peter Zijlstra
2016-01-15 17:46 ` Paul E. McKenney
2016-01-15 21:27 ` Peter Zijlstra
2016-01-15 21:58 ` Paul E. McKenney
2016-01-25 16:42 ` Will Deacon
2016-01-26 6:03 ` Paul E. McKenney
2016-01-26 10:19 ` Peter Zijlstra
2016-01-26 20:13 ` Paul E. McKenney
2016-01-27 8:39 ` Peter Zijlstra
2016-01-26 12:16 ` Will Deacon
2016-01-26 14:35 ` Boqun Feng
2016-01-26 19:58 ` Paul E. McKenney [this message]
2016-01-27 10:25 ` Will Deacon
2016-01-27 23:32 ` Paul E. McKenney
2016-01-15 17:39 ` Paul E. McKenney
2016-01-15 21:29 ` Peter Zijlstra
2016-01-15 22:01 ` Paul E. McKenney
2016-01-25 18:02 ` Will Deacon
2016-01-26 6:12 ` Paul E. McKenney
2016-01-26 10:15 ` Peter Zijlstra
2016-01-10 14:18 ` [PATCH v3 12/41] x86/um: " Michael S. Tsirkin
2016-01-10 14:18 ` [PATCH v3 13/41] x86: " Michael S. Tsirkin
2016-01-12 14:10 ` Thomas Gleixner
2016-01-10 14:18 ` [PATCH v3 14/41] asm-generic: add __smp_xxx wrappers Michael S. Tsirkin
2016-01-10 14:18 ` [PATCH v3 15/41] powerpc: define __smp_xxx Michael S. Tsirkin
2016-01-10 14:18 ` [PATCH v3 16/41] arm64: " Michael S. Tsirkin
2016-01-10 14:18 ` [PATCH v3 17/41] arm: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 18/41] blackfin: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 19/41] ia64: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 20/41] metag: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 21/41] mips: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 22/41] s390: " Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 23/41] sh: define __smp_xxx, fix smp_store_mb for !SMP Michael S. Tsirkin
2016-01-10 14:19 ` [PATCH v3 24/41] sparc: define __smp_xxx Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 25/41] tile: " Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 26/41] xtensa: " Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 27/41] x86: " Michael S. Tsirkin
2016-01-12 14:11 ` Thomas Gleixner
2016-01-10 14:20 ` [PATCH v3 28/41] asm-generic: implement virt_xxx memory barriers Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 29/41] Revert "virtio_ring: Update weak barriers to use dma_wmb/rmb" Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 30/41] virtio_ring: update weak barriers to use virt_xxx Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 31/41] sh: support 1 and 2 byte xchg Michael S. Tsirkin
2016-01-10 14:20 ` [PATCH v3 32/41] sh: move xchg_cmpxchg to a header by itself Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 33/41] virtio_ring: use virt_store_mb Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 34/41] checkpatch.pl: add missing memory barriers Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 35/41] checkpatch: check for __smp outside barrier.h Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 36/41] checkpatch: add virt barriers Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 37/41] xenbus: use virt_xxx barriers Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 38/41] xen/io: " Michael S. Tsirkin
2016-01-10 14:21 ` [PATCH v3 39/41] xen/events: " Michael S. Tsirkin
2016-01-11 11:12 ` David Vrabel
2016-01-10 14:22 ` [PATCH v3 40/41] s390: use generic memory barriers Michael S. Tsirkin
2016-01-10 14:22 ` [PATCH v3 41/41] s390: more efficient smp barriers Michael S. Tsirkin
2016-01-12 12:50 ` [PATCH v3 00/41] arch: barrier cleanup + barriers for virt Peter Zijlstra
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