From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will.deacon@arm.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Boqun Feng <boqun.feng@gmail.com>
Cc: <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
"Thomas Gleixner" <tglx@linutronix.de>, <rkuo@codeaurora.org>,
<james.hogan@imgtec.com>, <jejb@parisc-linux.org>,
<davem@davemloft.net>, <cmetcalf@mellanox.com>,
arcml <linux-snps-arc@lists.infradead.org>
Subject: Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny' architectures
Date: Fri, 9 Jun 2017 10:28:50 -0700 [thread overview]
Message-ID: <ecffd3a6-167b-c0ed-0121-1a3a4141f799@synopsys.com> (raw)
In-Reply-To: <20170609111305.bn4ca4uscbp6pgxn@hirez.programming.kicks-ass.net>
On 06/09/2017 04:13 AM, Peter Zijlstra wrote:
> On Fri, Jun 09, 2017 at 01:05:06PM +0200, Peter Zijlstra wrote:
>
>> The spinlock based atomics should be SC, that is, none of them appear to
>> place extra barriers in atomic_cmpxchg() or any of the other SC atomic
>> primitives and therefore seem to rely on their spinlock implementation
>> being SC (I did not fully validate all that).
>
> So I did see that ARC and PARISC have 'superfluous' smp_mb() calls
> around their spinlock implementation.
>
> That is, for spinlock semantics you only need one _after_ lock and one
> _before_ unlock. But the atomic stuff relies on being SC and thus would
> need one before and after both lock and unlock.
Right we discussed this a while back: https://lkml.org/lkml/2015/6/11/276
At the time when I tried removing these extra barriers, hackbench regressed. I'm
about to get a new quad core 1GHz chip (vs. the FPGA before) and will
re-experiment. Likely we don't need it otherwise I will add a comment of this
"feature"
> But ARC could probably optimize (if they still care about that hardware)
> by pulling out those barriers and putting it in the atomic
> implementation.
A bit confused here. Reading the lkml posting for this thread, you posted 2
patches, and they had to do with atomic_set() for EZChip platform which is really
special (no ll/sc). The extra smp_mb() is related to ll/sc variants. Just tryign
to make sure that we are talking 2 different things here :-)
-Vineet
WARNING: multiple messages have this Message-ID (diff)
From: Vineet.Gupta1@synopsys.com (Vineet Gupta)
To: linux-snps-arc@lists.infradead.org
Subject: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny' architectures
Date: Fri, 9 Jun 2017 10:28:50 -0700 [thread overview]
Message-ID: <ecffd3a6-167b-c0ed-0121-1a3a4141f799@synopsys.com> (raw)
In-Reply-To: <20170609111305.bn4ca4uscbp6pgxn@hirez.programming.kicks-ass.net>
On 06/09/2017 04:13 AM, Peter Zijlstra wrote:
> On Fri, Jun 09, 2017@01:05:06PM +0200, Peter Zijlstra wrote:
>
>> The spinlock based atomics should be SC, that is, none of them appear to
>> place extra barriers in atomic_cmpxchg() or any of the other SC atomic
>> primitives and therefore seem to rely on their spinlock implementation
>> being SC (I did not fully validate all that).
>
> So I did see that ARC and PARISC have 'superfluous' smp_mb() calls
> around their spinlock implementation.
>
> That is, for spinlock semantics you only need one _after_ lock and one
> _before_ unlock. But the atomic stuff relies on being SC and thus would
> need one before and after both lock and unlock.
Right we discussed this a while back: https://lkml.org/lkml/2015/6/11/276
At the time when I tried removing these extra barriers, hackbench regressed. I'm
about to get a new quad core 1GHz chip (vs. the FPGA before) and will
re-experiment. Likely we don't need it otherwise I will add a comment of this
"feature"
> But ARC could probably optimize (if they still care about that hardware)
> by pulling out those barriers and putting it in the atomic
> implementation.
A bit confused here. Reading the lkml posting for this thread, you posted 2
patches, and they had to do with atomic_set() for EZChip platform which is really
special (no ll/sc). The extra smp_mb() is related to ll/sc variants. Just tryign
to make sure that we are talking 2 different things here :-)
-Vineet
next prev parent reply other threads:[~2017-06-09 17:29 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-09 9:24 [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Peter Zijlstra
2017-06-09 11:05 ` [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny' architectures Peter Zijlstra
2017-06-09 11:13 ` Peter Zijlstra
2017-06-09 17:28 ` Vineet Gupta [this message]
2017-06-09 17:28 ` Vineet Gupta
2017-06-09 18:49 ` Peter Zijlstra
2017-06-09 18:49 ` Peter Zijlstra
2017-06-09 18:58 ` James Bottomley
2017-06-09 14:03 ` Chris Metcalf
2017-08-10 12:10 ` [tip:locking/core] locking/atomic: " tip-bot for Peter Zijlstra
2017-06-09 15:44 ` [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Will Deacon
2017-06-09 19:36 ` Peter Zijlstra
2017-06-11 13:56 ` Boqun Feng
2017-06-12 14:49 ` Peter Zijlstra
2017-06-13 6:39 ` Boqun Feng
2017-06-14 12:33 ` Will Deacon
2017-07-12 12:53 ` Boqun Feng
2017-07-12 13:08 ` Peter Zijlstra
2017-07-12 19:13 ` Paul E. McKenney
2017-07-26 11:53 ` [RFC][PATCH v3]: documentation,atomic: Add new documents Peter Zijlstra
2017-07-26 12:47 ` Boqun Feng
2017-07-31 9:05 ` Peter Zijlstra
2017-07-31 11:04 ` Boqun Feng
2017-07-31 17:43 ` Paul E. McKenney
2017-08-01 2:14 ` Boqun Feng
2017-08-01 9:01 ` Peter Zijlstra
2017-08-01 10:19 ` Will Deacon
2017-08-01 11:47 ` Peter Zijlstra
2017-08-01 12:17 ` Will Deacon
2017-08-01 12:52 ` Peter Zijlstra
2017-08-01 16:14 ` Paul E. McKenney
2017-08-01 16:42 ` Peter Zijlstra
2017-08-01 16:53 ` Will Deacon
2017-08-01 22:18 ` Paul E. McKenney
2017-08-02 8:46 ` Will Deacon
2017-08-01 18:37 ` Paul E. McKenney
2017-08-02 9:45 ` Will Deacon
2017-08-02 16:17 ` Paul E. McKenney
2017-08-03 14:05 ` Boqun Feng
2017-08-03 14:55 ` Paul E. McKenney
2017-08-03 16:12 ` Will Deacon
2017-08-03 16:58 ` Paul E. McKenney
2017-08-01 13:35 ` Paul E. McKenney
2017-07-26 16:28 ` Randy Dunlap
2017-06-09 18:15 ` [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Randy Dunlap
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