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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	andrea.parri@amarulasolutions.com,
	Will Deacon <will.deacon@arm.com>,
	Akira Yokosawa <akiyks@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nick Piggin <npiggin@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Tue, 17 Jul 2018 11:44:23 -0700
Message-ID: <CA+55aFwA=TmJJkKvskX3wL9Rs1j8xZJDJVghv=G0spYufK9kYg@mail.gmail.com> (raw)
In-Reply-To: <20180717183341.GQ12945@linux.vnet.ibm.com>

On Tue, Jul 17, 2018 at 11:31 AM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> The isync provides ordering roughly similar to lwsync, but nowhere near
> as strong as sync, and it is sync that would be needed to cause lock
> acquisition to provide full ordering.

That's only true when looking at isync in isolation.

Read the part I quoted. The AIX documentation implies that the
*sequence* of load-compare-conditional branch-isync is a memory
barrier, even if isync on its own is now.

So I'm just saying that

 (a) isync-on-lock is supposed to be much cheaper than sync-on-lock

 (b) the AIX documentation at least implies that isync-on-lock (when
used together the the whole locking sequence) is actually a memory
barrier

Now, admittedly the powerpc barrier instructions are unfathomable
crazy stuff, so who knows. But:

 (a) lwsync is a memory barrier for all the "easy" cases (ie
load->store, load->load, and store->load).

 (b) lwsync is *not* a memory barrier for the store->load case.

 (c) isync *is* (when in that *sequence*) a memory barrier for a
store->load case (and has to be: loads inside a spinlocked region MUST
NOT be done earlier than stores outside of it!).

So a unlock/lock sequence where the unlock is using lwsync, and the
lock is using isync, should in fact be a full memory barrier (which is
the semantics we're looking for).

So doing performance testing on sync/lwsync (for lock/unlock
respectively) seems the wrong thing to do.  Please test the
isync/lwsync case instead.

Hmm? What am I missing?

              Linus

  parent reply index

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-09 20:01 Alan Stern
2018-07-09 21:45 ` Paul E. McKenney
2018-07-10 13:57   ` Alan Stern
2018-07-10 16:25     ` Paul E. McKenney
     [not found]       ` <Pine.LNX.4.44L0.1807101416390.1449-100000@iolanthe.rowland.org>
2018-07-10 19:58         ` [PATCH v3] " Paul E. McKenney
2018-07-10 20:24           ` Alan Stern
2018-07-10 20:31             ` Paul E. McKenney
2018-07-11  9:43         ` Will Deacon
2018-07-11 15:42           ` Paul E. McKenney
2018-07-11 16:17             ` Andrea Parri
2018-07-11 18:03               ` Paul E. McKenney
2018-07-11 16:34           ` Peter Zijlstra
2018-07-11 18:10             ` Paul E. McKenney
2018-07-10  9:38 ` [PATCH v2] " Andrea Parri
2018-07-10 14:48   ` Alan Stern
2018-07-10 15:24     ` Andrea Parri
2018-07-10 15:34       ` Alan Stern
2018-07-10 23:14         ` Andrea Parri
2018-07-11  9:43   ` Will Deacon
2018-07-11 12:34     ` Andrea Parri
2018-07-11 12:54       ` Andrea Parri
2018-07-11 15:57       ` Will Deacon
2018-07-11 16:28         ` Andrea Parri
2018-07-11 17:00         ` Peter Zijlstra
2018-07-11 17:50           ` Daniel Lustig
2018-07-12  8:34             ` Andrea Parri
2018-07-12  9:29             ` Peter Zijlstra
2018-07-12  7:40       ` Peter Zijlstra
2018-07-12  9:34         ` Peter Zijlstra
2018-07-12  9:45           ` Will Deacon
2018-07-13  2:17             ` Daniel Lustig
2018-07-12 11:52         ` Andrea Parri
2018-07-12 12:01           ` Andrea Parri
2018-07-12 12:11             ` Peter Zijlstra
2018-07-12 13:48           ` Peter Zijlstra
2018-07-12 16:19             ` Paul E. McKenney
2018-07-12 17:04             ` Alan Stern
2018-07-12 17:14               ` Will Deacon
2018-07-12 17:28               ` Paul E. McKenney
2018-07-12 18:05                 ` Peter Zijlstra
2018-07-12 18:10                   ` Linus Torvalds
2018-07-12 19:52                     ` Andrea Parri
2018-07-12 20:24                       ` Andrea Parri
2018-07-13  2:05                     ` Daniel Lustig
2018-07-13  4:03                       ` Paul E. McKenney
2018-07-13  9:07                       ` Andrea Parri
2018-07-13  9:35                         ` Will Deacon
2018-07-13 17:16                           ` Linus Torvalds
2018-07-13 19:06                             ` Andrea Parri
2018-07-14  1:51                               ` Alan Stern
2018-07-14  2:58                                 ` Linus Torvalds
2018-07-16  2:31                                   ` Paul E. McKenney
2018-07-13 11:08                     ` Peter Zijlstra
2018-07-13 13:15                       ` Michael Ellerman
2018-07-13 16:42                         ` Peter Zijlstra
2018-07-13 19:56                           ` Andrea Parri
2018-07-16 14:40                           ` Michael Ellerman
2018-07-16 19:01                             ` Peter Zijlstra
2018-07-16 19:30                             ` Linus Torvalds
2018-07-17 14:45                               ` Michael Ellerman
2018-07-17 16:19                                 ` Linus Torvalds
2018-07-17 18:33                                   ` Paul E. McKenney
2018-07-17 18:42                                     ` Peter Zijlstra
2018-07-17 19:40                                       ` Paul E. McKenney
2018-07-17 19:47                                       ` Alan Stern
2018-07-17 18:44                                     ` Linus Torvalds [this message]
2018-07-17 18:49                                       ` Linus Torvalds
2018-07-17 19:42                                         ` Paul E. McKenney
2018-07-17 19:37                                       ` Alan Stern
2018-07-17 20:13                                         ` Linus Torvalds
2018-07-17 19:38                                       ` Paul E. McKenney
2018-07-17 19:40                                     ` Andrea Parri
2018-07-17 19:52                                       ` Paul E. McKenney
2018-07-18 12:31                                   ` Michael Ellerman
2018-07-18 13:16                             ` Michael Ellerman
2018-07-12 17:52               ` Andrea Parri
2018-07-12 20:43                 ` Alan Stern
2018-07-12 21:13                   ` Andrea Parri
2018-07-12 21:23                     ` Andrea Parri
2018-07-12 18:33               ` Peter Zijlstra
2018-07-12 17:45             ` Andrea Parri
2018-07-10 16:56 ` Daniel Lustig
     [not found]   ` <Pine.LNX.4.44L0.1807101315140.1449-100000@iolanthe.rowland.org>
2018-07-10 23:31     ` Andrea Parri
2018-07-11 14:19       ` Alan Stern

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