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

[Will Deacon <will.deacon@arm.com>]

Hi Ben,

On Wed, Aug 15, 2012 at 05:29:26AM +0100, Ben Hutchings wrote:
> On Mon, 2012-08-13 at 15:13 -0700, Greg Kroah-Hartman wrote:
> > From: Greg KH <gregkh@linuxfoundation.org>
> > 
> > 3.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Will Deacon <will.deacon@arm.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?

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.

Will