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


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