Messages in this thread | | | Date | Wed, 15 Aug 2012 10:10:29 +0100 | From | Will Deacon <> | Subject | Re: [ 10/65] ARM: 7467/1: mutex: use generic xchg-based implementation for ARMv6+ |
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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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