linux-kernel - Re: [ 10/65] ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+

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Message-ID: <1345004966.824.214.camel@deadeye.wl.decadent.org.uk>
Date:	Wed, 15 Aug 2012 05:29:26 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Will Deacon <will.deacon@....com>,
	Russell King <rmk+kernel@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nico@...aro.org>,
	Shan Kang <kangshan0910@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [ 10/65] ARM: 7467/1: mutex: use generic xchg-based
 implementation for ARMv6+

On Mon, 2012-08-13 at 15:13 -0700, Greg Kroah-Hartman wrote:
> From: Greg KH <gregkh@...uxfoundation.org>
> 
> 3.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Will Deacon <will.deacon@....com>
> 
> commit a76d7bd96d65fa5119adba97e1b58d95f2e78829 upstream.
> 
> The open-coded mutex implementation for ARMv6+ cores suffers from a
> severe lack of barriers, so in the uncontended case we don't actually
> protect any accesses performed during the critical section.
> 
> Furthermore, the code is largely a duplication of the ARMv6+ atomic_dec
> code but optimised to remove a branch instruction, as the mutex fastpath
> was previously inlined. Now that this is executed out-of-line, we can
> reuse the atomic access code for the locking (in fact, we use the xchg
> code as this produces shorter critical sections).
> 
> This patch uses the generic xchg based implementation for mutexes on
> ARMv6+, which introduces barriers to the lock/unlock operations and also
> has the benefit of removing a fair amount of inline assembly code.
[...]

I understand that a further fix is needed on top of this
<http://article.gmane.org/gmane.linux.ports.arm.kernel/181693> but it's
not in Linus's tree yet.  Is it better to apply this on its own or to
wait for the complete fix?

Ben.

-- 
Ben Hutchings
I say we take off; nuke the site from orbit.  It's the only way to be sure.

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