Re: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h

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

On Tue, Aug 07, 2012 at 02:48:42PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-08-07 at 12:56 +0100, Will Deacon wrote:
> > ARM recently moved to asm-generic/mutex-xchg.h for its mutex implementation
> > after our previous implementation was found to be missing some crucial
> > memory barriers. 
> 
> 
> This is a76d7bd96d ("ARM: 7467/1: mutex: use generic xchg-based
> implementation for ARMv6+"), right? Why do you use xchg and not dec
> based? The changelog mumbles something about shorter critical sections,
> but me not knowing anything about ARM wonders about the why of that.

Correct, that's the patch. We don't have atomic add/sub instructions on ARM,
so instead we have to do:

1:	ldrex	...	@ Exclusive load
	add/sub ...     @ Do the arithmetic
	strex	...	@ Exclusive store
	cmp	...	@ Check the store succeeded
	bne	1b	@ Retry if we weren't atomic

So using dec adds a sub where we wouldn't need an instruction there for xchg.
I suspect there's no measurable difference between the two, but we use the
xchg-based implementation for CPUs prior to ARMv6 so it saves an ifdef as
well. Some discussion on the original patch here:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2012-July/109333.html

> >         Task A        Task B        Task C        Lock value
> > 0                                                     1
> > 1       lock()                                        0
> > 2                     lock()                          0
> > 3                     spin(A)                         0
> > 4       unlock()                                      1
> > 5                                   lock()            0
> > 6                     cmpxchg(1,0)                    0
> > 7                     contended()                    -1
> > 8       lock()                                        0
> > 9       spin(C)                                       0
> > 10                                  unlock()          1
> > 11      cmpxchg(1,0)                                  0
> > 12      unlock()                                      1
> > 
> > 
> > At this point, the lock is unlocked, but Task B is in an uninterruptible
> > sleep with nobody to wake it up.

[...]

> But in this case, either B is still spinning in our spin-loop, or it has
> already passed the atomic_xchg(&lock->count, -1) when we fell out.

Yes, it does that xchg on line 7 (see the lock value of -1)...

> Since you say B is in UNINTERRUPTIBLE state, we'll assume it fell
> through and so the lock count should be -1 (or less) to mark it
> contended.

... but then A sets it straight back to 0 in __mutex_fastpath_lock and falls
down the slowpath due to it being contended. The problem is that it doesn't
restore the -1 when it acquires the lock on line 11, so B is never woken up.

Will