Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

Davidlohr Bueso
2014-06-05T01:10:38Z

On Wed, 2014-06-04 at 14:47 -0700, Jason Low wrote:
> On Wed, 2014-06-04 at 13:28 -0700, Davidlohr Bueso wrote:
> > On Wed, 2014-06-04 at 12:08 -0700, Jason Low wrote:
> > > In __mutex_trylock_slowpath(), we acquire the wait_lock spinlock,
> > > xchg() lock->count with -1, then set lock->count back to 0 if there
> > > are no waiters, and return true if the prev lock count was 1.
> > >
> > > However, if we the mutex is already locked, then there may not be
> > ^^ leave that out.
> >
> > > much point in attempting the above operations.
> >
> > Isn't this redundant? I mean, if we enter the slowpath its because
> > __mutex_fastpath_trylock() already failed so we already know that the
> > lock is taken.
>
> This function is really just used as an alternative method of trylock
> for !__HAVE_ARCH_CMPXCHG. In that case, the fastpath can call directly
> into the slowpath function, without checking for if the lock is taken.

Ah, ok I hadn't seen that we do this in 32bit x86 and was wondering why
the heck we fallback on a slowpath for something like trylock, which
should return right away no matter what. I'd suggest explicitly
mentioning this in the changelog. Otherwise makes sense now.