From: Will Deacon <will.deacon@arm.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Waiman Long <longman@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] locking: Remove an insn from spin and write locks
Date: Tue, 21 Aug 2018 02:54:15 +0100 [thread overview]
Message-ID: <20180821015413.GD1758@brain-police> (raw)
In-Reply-To: <20180820162639.GC25153@bombadil.infradead.org>
On Mon, Aug 20, 2018 at 09:26:39AM -0700, Matthew Wilcox wrote:
> On Mon, Aug 20, 2018 at 05:56:50PM +0200, Peter Zijlstra wrote:
> > Yeah, _acquire should be retained; sorry about loosing that. I'm neck
> > deep into tlb invalidate stuff and wrote this without much thinking
> > involved.
>
> NP. Here's the current version I've got, with some updated likely()
> hints.
>
> From 337298a88266f7b21492f893c2bf05409a5392c8 Mon Sep 17 00:00:00 2001
> From: Matthew Wilcox <willy@infradead.org>
> Date: Mon, 20 Aug 2018 10:19:14 -0400
> Subject: [PATCH] locking: Remove an insn from spin and write locks
>
> Both spin locks and write locks currently do:
>
> f0 0f b1 17 lock cmpxchg %edx,(%rdi)
> 85 c0 test %eax,%eax
> 75 05 jne [slowpath]
>
> This 'test' insn is superfluous; the cmpxchg insn sets the Z flag
> appropriately. Peter pointed out that using atomic_try_cmpxchg_acquire()
> will let the compiler know this is true. Comparing before/after
> disassemblies show the only effect is to remove this insn.
>
> Take this opportunity to make the spin & write lock code resemble each
> other more closely and have similar likely() hints.
>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Matthew Wilcox <willy@infradead.org>
> ---
> include/asm-generic/qrwlock.h | 7 ++++---
> include/asm-generic/qspinlock.h | 17 ++++++++++-------
> 2 files changed, 14 insertions(+), 10 deletions(-)
Shouldn't make any difference on arm64, so:
Acked-by: Will Deacon <will.deacon@arm.com>
Will
next prev parent reply other threads:[~2018-08-21 1:54 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-20 15:06 [PATCH] locking: Remove an insn from spin and write locks Matthew Wilcox
2018-08-20 15:14 ` Waiman Long
2018-08-20 15:50 ` Matthew Wilcox
2018-08-20 15:55 ` Waiman Long
2018-08-20 15:56 ` Peter Zijlstra
2018-08-20 16:26 ` Matthew Wilcox
2018-08-21 1:54 ` Will Deacon [this message]
2018-10-02 10:07 ` [tip:locking/core] locking/spinlocks: Remove an instruction " tip-bot for Matthew Wilcox
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