From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>,
Peter Zijlstra <peterz@infradead.org>,
"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>,
David Miller <davem@davemloft.net>,
linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-s390@vger.kernel.org, sparclinux@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-metag@vger.kernel.org, linux-mips@linux-mips.org,
x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net,
adi-buildroot-devel@lists.sourceforge.net,
linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org,
xen-devel@lists.xenproject.org,
Ralf Baechle <ralf@linux-mips.org>,
Ingo Molnar <mingo@kernel.org>,
ddaney.cavm@gmail.com, james.hogan@imgtec.com,
Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h
Date: Fri, 15 Jan 2016 11:28:45 -0800 [thread overview]
Message-ID: <20160115192845.GA12510@linux.vnet.ibm.com> (raw)
In-Reply-To: <20160115175401.GW3818@linux.vnet.ibm.com>
On Fri, Jan 15, 2016 at 09:54:01AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 15, 2016 at 10:24:32AM +0000, Will Deacon wrote:
> > On Thu, Jan 14, 2016 at 02:55:10PM -0800, Paul E. McKenney wrote:
> > > On Thu, Jan 14, 2016 at 01:36:50PM -0800, Leonid Yegoshin wrote:
> > > > On 01/14/2016 01:29 PM, Paul E. McKenney wrote:
> > > > >
> > > > >>On 01/14/2016 12:34 PM, Paul E. McKenney wrote:
> > > > >>>
> > > > >>>The WRC+addr+addr is OK because data dependencies are not required to be
> > > > >>>transitive, in other words, they are not required to flow from one CPU to
> > > > >>>another without the help of an explicit memory barrier.
> > > > >>I don't see any reliable way to fit WRC+addr+addr into "DATA
> > > > >>DEPENDENCY BARRIERS" section recommendation to have data dependency
> > > > >>barrier between read of a shared pointer/index and read the shared
> > > > >>data based on that pointer. If you have this two reads, it doesn't
> > > > >>matter the rest of scenario, you should put the dependency barrier
> > > > >>in code anyway. If you don't do it in WRC+addr+addr scenario then
> > > > >>after years it can be easily changed to different scenario which
> > > > >>fits some of scenario in "DATA DEPENDENCY BARRIERS" section and
> > > > >>fails.
> > > > >The trick is that lockless_dereference() contains an
> > > > >smp_read_barrier_depends():
> > > > >
> > > > >#define lockless_dereference(p) \
> > > > >({ \
> > > > > typeof(p) _________p1 = READ_ONCE(p); \
> > > > > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> > > > > (_________p1); \
> > > > >})
> > > > >
> > > > >Or am I missing your point?
> > > >
> > > > WRC+addr+addr has no any barrier. lockless_dereference() has a
> > > > barrier. I don't see a common points between this and that in your
> > > > answer, sorry.
> > >
> > > Me, I am wondering what WRC+addr+addr has to do with anything at all.
> >
> > See my earlier reply [1] (but also, your WRC Linux example looks more
> > like a variant on WWC and I couldn't really follow it).
>
> I will revisit my WRC Linux example. And yes, creating litmus tests
> that use non-fake dependencies is still a bit of an undertaking. :-/
> I am sure that it will seem more natural with time and experience...
Hmmm... You are quite right, I did do WWC. I need to change cpu2()'s
last access from a store to a load to get WRC. Plus the levels of
indirection definitely didn't match up, did they?
struct foo {
struct foo *next;
};
struct foo a;
struct foo b;
struct foo c = { &a };
struct foo d = { &b };
struct foo x = { &c };
struct foo y = { &d };
struct foo *r1, *r2, *r3;
void cpu0(void)
{
WRITE_ONCE(x.next, &y);
}
void cpu1(void)
{
r1 = lockless_dereference(x.next);
WRITE_ONCE(r1->next, &x);
}
void cpu2(void)
{
r2 = lockless_dereference(y.next);
r3 = READ_ONCE(r2->next);
}
In this case, it is legal to end the run with:
r1 == &y && r2 == &x && r3 == &c
Please see below for a ppcmem litmus test.
So, did I get it right this time? ;-)
Thanx, Paul
PS. And yes, working through this does help me understand the
benefits of fake dependencies. Why do you ask? ;-)
------------------------------------------------------------------------
PPC WRCnf+addrs
""
{
0:r2=x; 0:r3=y;
1:r2=x; 1:r3=y;
2:r2=x; 2:r3=y;
c=a; d=b; x=c; y=d;
}
P0 | P1 | P2 ;
stw r3,0(r2) | lwz r8,0(r2) | lwz r8,0(r3) ;
| stw r2,0(r3) | lwz r9,0(r8) ;
exists
(1:r8=y /\ 2:r8=x /\ 2:r9=c)
next prev parent reply other threads:[~2016-01-15 19:29 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 [this message]
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
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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