From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Ingo Molnar <mingo@kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will.deacon@arm.com>,
Waiman Long <waiman.long@hp.com>
Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants
Date: Thu, 1 Oct 2015 08:09:09 -0700 [thread overview]
Message-ID: <20151001150909.GC4043@linux.vnet.ibm.com> (raw)
In-Reply-To: <20151001122440.GP2881@worktop.programming.kicks-ass.net>
On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 16, 2015 at 11:49:32PM +0800, Boqun Feng wrote:
> > Implement xchg_relaxed and define atomic{,64}_xchg_* as xchg_relaxed,
> > based on these _relaxed variants, release/acquire variants can be built.
> >
> > Note that xchg_relaxed and atomic_{,64}_xchg_relaxed are not compiler
> > barriers.
>
> Hmm, and I note your previous patch creating the regular _relaxed
> thingies also removes the memory clobber.
>
> And looking at the ARM _relaxed patch from Will, I see their _relaxed
> ops are also not a compiler barrier.
>
> I must say I'm somewhat surprised by this level of relaxation, I had
> expected to only loose SMP barriers, not the program order ones.
>
> Is there a good argument for this?
Yes, when we say "relaxed", we really mean relaxed. ;-)
Both the CPU and the compiler are allowed to reorder around relaxed
operations.
Thanx, Paul
next prev parent reply other threads:[~2015-10-01 15:09 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-16 15:49 [RFC v2 0/7] atomics: powerpc: Implement relaxed/acquire/release variants of some atomics Boqun Feng
2015-09-16 15:49 ` [RFC v2 1/7] atomics: Add test for atomic operations with _relaxed variants Boqun Feng
2015-10-12 9:30 ` Will Deacon
2015-10-12 9:38 ` Boqun Feng
2015-09-16 15:49 ` [RFC v2 2/7] atomics: Allow architectures to define their own __atomic_op_* helpers Boqun Feng
2015-09-16 15:49 ` [RFC v2 3/7] powerpc: atomic: Implement atomic{, 64}_{add, sub}_return_* variants Boqun Feng
2015-09-18 16:59 ` [RFC v2 3/7] powerpc: atomic: Implement atomic{,64}_{add,sub}_return_* variants Will Deacon
2015-09-19 15:33 ` Boqun Feng
2015-09-20 8:23 ` Boqun Feng
2015-09-21 22:24 ` Will Deacon
2015-09-21 23:26 ` Boqun Feng
2015-09-21 23:37 ` Boqun Feng
2015-09-22 15:25 ` Paul E. McKenney
2015-09-23 0:07 ` Boqun Feng
2015-09-25 21:29 ` Paul E. McKenney
2015-09-26 2:18 ` Boqun Feng
2015-09-16 15:49 ` [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{, 64}_xchg_* variants Boqun Feng
2015-10-01 12:24 ` [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants Peter Zijlstra
2015-10-01 15:09 ` Paul E. McKenney [this message]
2015-10-01 17:13 ` Peter Zijlstra
2015-10-01 18:03 ` Paul E. McKenney
2015-10-01 18:23 ` Peter Zijlstra
2015-10-01 19:41 ` Paul E. McKenney
2015-10-05 14:44 ` Will Deacon
2015-10-05 16:57 ` Paul E. McKenney
2015-10-12 1:17 ` Boqun Feng
2015-10-12 9:28 ` Will Deacon
2015-10-12 23:24 ` Paul E. McKenney
2015-09-16 15:49 ` [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{, 64}_* and atomic{, 64}_cmpxchg_* variants Boqun Feng
2015-10-01 12:27 ` [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants Peter Zijlstra
2015-10-01 12:36 ` Peter Zijlstra
2015-10-01 15:12 ` Paul E. McKenney
2015-10-01 17:11 ` Peter Zijlstra
2015-10-01 15:13 ` Paul E. McKenney
2015-10-10 1:58 ` Boqun Feng
2015-10-11 10:25 ` Boqun Feng
2015-10-12 6:46 ` Peter Zijlstra
2015-10-12 7:03 ` Boqun Feng
2015-09-16 15:49 ` [RFC v2 6/7] powerpc: atomic: Make atomic{, 64}_xchg and xchg a full barrier Boqun Feng
2015-10-01 12:28 ` [RFC v2 6/7] powerpc: atomic: Make atomic{,64}_xchg " Peter Zijlstra
2015-10-01 23:19 ` Boqun Feng
2015-10-02 5:25 ` Peter Zijlstra
2015-09-16 15:49 ` [RFC v2 7/7] powerpc: atomic: Make atomic{, 64}_cmpxchg and cmpxchg " Boqun Feng
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