linux-kernel - Re: [ 20/82] ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+

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Message-ID: <20120815140808.GD22871@kroah.com>
Date:	Wed, 15 Aug 2012 07:08:08 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Ben Hutchings <ben@...adent.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>,
	Will Deacon <will.deacon@....com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: [ 20/82] ARM: 7467/1: mutex: use generic xchg-based
 implementation for ARMv6+

On Wed, Aug 15, 2012 at 02:56:22PM +0100, Ben Hutchings wrote:
> On Mon, 2012-08-13 at 13:18 -0700, Greg Kroah-Hartman wrote:
> > From: Greg KH <gregkh@...uxfoundation.org>
> > 
> > 3.5-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.
> [...]
> 
> Here also, I think this should be deferred.

"also"?  Am I missing some context here?  Why should we deferr this one?
What do we need to wait for?

confused,

greg k-h
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