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

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Message-ID: <1345039901.824.221.camel@deadeye.wl.decadent.org.uk>
Date:	Wed, 15 Aug 2012 15:11:41 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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, 2012-08-15 at 07:08 -0700, Greg Kroah-Hartman wrote:
> 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?

This is the same as 3.4.9-rc1 patch 10/65, which I queried as it
apparently caused a regression.

Will Deacon wrote:
> The additional patch should also be CC'd to stable and is sitting in -tip
> somewhere I believe, so it shouldn't be long before it does hit mainline.
> 
> Without this patch there's a memory-ordering bug (which we seem to have hit
> once in > 5 years). With the patch there's a mutex lockup issue on SMP systems
> that I can provoke with enough hackbenching, so you may want to hold off for
> now.

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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