Message-ID: <1345039364.824.217.camel@deadeye.wl.decadent.org.uk>
Subject: Re: [ 06/44] ARM: 7467/1: mutex: use generic xchg-based
 implementation for ARMv6+
From: Ben Hutchings <ben@decadent.org.uk>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org,
        alan@lxorguk.ukuu.org.uk, Arnd Bergmann <arnd@arndb.de>,
        Nicolas Pitre <nico@linaro.org>, Shan Kang <kangshan0910@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <rmk+kernel@arm.linux.org.uk>
Date: Wed, 15 Aug 2012 15:02:44 +0100
In-Reply-To: <20120813220142.690510091@linuxfoundation.org>
References: <20120813220142.113186818@linuxfoundation.org>
	 <20120813220142.690510091@linuxfoundation.org>
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On Mon, 2012-08-13 at 15:02 -0700, Greg Kroah-Hartman wrote:
> From: Greg KH <gregkh@linuxfoundation.org>
>=20
> 3.0-stable review patch.  If anyone has any objections, please let me kno=
w.
>=20
> ------------------
>=20
> From: Will Deacon <will.deacon@arm.com>
>=20
> commit a76d7bd96d65fa5119adba97e1b58d95f2e78829 upstream.
>=20
> 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.
>=20
> 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).
>=20
> 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.
[...]

This should be deferred, same as in 3.4.y and 3.5.y.

Ben.

--=20
Ben Hutchings
I say we take off; nuke the site from orbit.  It's the only way to be sure.

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