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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
