From: David Laight <David.Laight@ACULAB.COM>
To: 'Peter Zijlstra' <peterz@infradead.org>,
"will.deacon@arm.com" <will.deacon@arm.com>,
"mingo@kernel.org" <mingo@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"longman@redhat.com" <longman@redhat.com>,
"andrea.parri@amarulasolutions.com"
<andrea.parri@amarulasolutions.com>,
"tglx@linutronix.de" <tglx@linutronix.de>
Subject: RE: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86
Date: Thu, 27 Sep 2018 12:16:14 +0000 [thread overview]
Message-ID: <ec232ce22c1a409180725d5e88a1175a@AcuMS.aculab.com> (raw)
In-Reply-To: <20180926111307.513429499@infradead.org>
From: Peter Zijlstra
> Sent: 26 September 2018 12:01
>
> On x86 we cannot do fetch_or with a single instruction and end up
> using a cmpxchg loop, this reduces determinism. Replace the fetch_or
> with a very tricky composite xchg8 + load.
>
> The basic idea is that we use xchg8 to test-and-set the pending bit
> (when it is a byte) and then a load to fetch the whole word. Using
> two instructions of course opens a window we previously did not have.
...
IIRC the load will be 'slow' because it will have to wait for the
earlier store to actually complete - rather than being satisfied
by data from the store buffer (because the widths are different).
This may not matter for xchg ?
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2018-09-27 12:14 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-26 11:01 [RFC][PATCH 0/3] locking/qspinlock: Improve determinism " Peter Zijlstra
2018-09-26 11:01 ` [RFC][PATCH 1/3] locking/qspinlock: Re-order code Peter Zijlstra
2018-10-01 17:17 ` Will Deacon
2018-09-26 11:01 ` [RFC][PATCH 2/3] locking/qspinlock: Rework some comments Peter Zijlstra
2018-10-01 17:17 ` Will Deacon
2018-10-01 19:10 ` Peter Zijlstra
2018-10-02 13:20 ` Will Deacon
2018-10-02 13:43 ` Peter Zijlstra
2018-09-26 11:01 ` [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86 Peter Zijlstra
2018-09-26 16:30 ` Waiman Long
2018-09-26 17:54 ` Peter Zijlstra
2018-09-27 7:29 ` Peter Zijlstra
2018-09-26 20:52 ` Andrea Parri
2018-09-27 7:17 ` Peter Zijlstra
2018-09-27 7:47 ` Andrea Parri
2018-09-27 7:59 ` Peter Zijlstra
2018-09-27 8:13 ` Andrea Parri
2018-09-27 8:57 ` Peter Zijlstra
2018-09-27 12:16 ` David Laight [this message]
2018-10-01 17:17 ` Will Deacon
2018-10-01 20:00 ` Peter Zijlstra
2018-10-02 13:19 ` Will Deacon
2018-10-02 14:14 ` Peter Zijlstra
2018-10-02 12:31 ` Andrea Parri
2018-10-02 13:22 ` Will Deacon
2018-10-02 13:44 ` Andrea Parri
2018-09-26 15:01 ` [RFC][PATCH 0/3] locking/qspinlock: Improve determinism " Sebastian Andrzej Siewior
2018-09-26 15:08 ` Thomas Gleixner
2018-09-26 15:38 ` Sebastian Andrzej Siewior
2018-09-26 16:20 ` Waiman Long
2018-09-26 17:51 ` Peter Zijlstra
2018-09-26 23:21 ` Waiman Long
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