[PATCH] ARM: mutex: use generic atomic_dec-based implementation for ARMv6+

[will.deacon@arm.com (Will Deacon)]

Hi Nico,

On Fri, Jul 13, 2012 at 02:21:41PM +0100, Nicolas Pitre wrote:
> On Fri, 13 Jul 2012, Will Deacon wrote:
> 
> > The open-coded mutex implementation for ARMv6+ cores suffers from a
> > couple of problems:
> > 
> > 	1. (major) There aren't any barriers in sight, so in the
> > 	   uncontended case we don't actually protect any accesses
> > 	   performed in during the critical section.
> > 
> > 	2. (minor) If the strex indicates failure to complete the store,
> > 	   we assume that the lock is contended and run away down the
> > 	   failure (slow) path. This assumption isn't correct and the
> > 	   core may fail the strex for reasons other than contention.
> > 
> > This patch solves both of these problems by using the generic atomic_dec
> > based implementation for mutexes on ARMv6+. This also has the benefit of
> > removing a fair amount of inline assembly code.
> 
> I don't agree with #2.  Mutexes should be optimized for the uncontended 
> case.  And in such case, strex failures are unlikely.

Hmm, my only argument here is that the architecture doesn't actually define
all the causes of such a failure, so assuming that they are unlikely is
really down to the CPU implementation. However, whilst I haven't benchmarked
the strex failure rate, it wouldn't make sense to fail them gratuitously
although we may still end up on the slow path for the uncontended case.

> There was a time where the fast path was inlined in the code while any 
> kind of contention processing was pushed out of line.  Going to the slow 
> path on strex failure just followed that model and provided correct 
> mutex behavior while making the inlined sequence one instruction 
> shorter.  Therefore #2 is not a problem at all, not even a minor one.

Ok, I wasn't aware of the history, thanks. The trade-off between size of
inlined code and possibly taking the slow path unnecessarily seems like a
compromise, so point (2) doesn't stand there...

> These days the whole mutex code is always out of line so the saving of a 
> single branch instruction in the whole kernel doesn't really matter 
> anymore. So to say that I agree with the patch but not the second half 
> of its justification.

... but like you say, the size of the out-of-line code doesn't matter as
much, so surely taking the slow patch for an uncontended mutex is a minor
issue here?

Anyway, that's an interesting discussion but I'll reword the commit message
so we can get this in while we ponder strex failures :)

How about:


    ARM: mutex: use generic atomic_dec-based implementation for ARMv6+

    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.

    This patch solves uses the generic atomic_dec 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.


Cheers,

Will