Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it is unlocked

Jason Low
2014-06-04T22:13:36Z

On Wed, 2014-06-04 at 23:54 +0200, Thomas Gleixner wrote:
> On Wed, 4 Jun 2014, Jason Low wrote:
> > On Wed, 2014-06-04 at 21:43 +0200, Peter Zijlstra wrote:
> > > > /*
> > > > * A negative mutex count indicates that waiters are sleeping waiting for the
> > > > - * mutex.
> > > > + * mutex, and a count of one indicates the mutex is unlocked.
> > > > */
> > > > #define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0)
> > > > +#define MUTEX_IS_UNLOCKED(mutex) (atomic_read(&(mutex)->count) == 1)
> > >
> > > So I recently saw that MUTEX_SHOW_NO_WAITER thing and cried a little;
> > > and now you're adding more of that same nonsense.
> > >
> > > Please make them inline functions, also can we rename the SHOW_NO_WAITER
> > > thing, because its not at all clear to me wtf it does; should it be
> > > called: mutex_no_waiters() or somesuch?
> >
> > Okay, I can make them inline functions. I mainly added the macro to keep
> > it consistent with the MUTEX_SHOW_NO_WAITER() check, but we can surely
>
> Consistency with a digusting and nonsensical macro is not really a
> good argument.

I agree :)

> > make this more clear. mutex_no_waiters() sounds fine, or perhaps
> > something like mutex_has_no_waiters()?
>
> Uuurg. So we end up with
>
> if (!mutex_has_no_waiters(m))
>
> if we check for waiters?
>
> Can we please go with the most intuitive thing:
>
> mutex_has_waiters()
>
> and have the callsites prepend the '!' in case they want to check
> there is no waiter?

Yes, !mutex_has_waiters() sounds like the better option to check for no
waiters. Same with using the already provided mutex_is_locked()
function.

> For heavens sake, we do not name macros/inlines in a way which fits
> the intended use case. We name them so they make sense.
>
> Your change log blurbs about readability. I have no idea what your
> understandig of readability is, but neither MUTEX_SHOW_NO_WAITERS nor
> mutex_has_no_waiters qualify for me. Ditto for MUTEX_IS_UNLOCKED.
>
> Care to look at the other lock implementations:
>
> rt_mutex_has_waiters()
> spin_is_locked()
> ....
>
> Why would it make sense to come up with reverse conventions for mutex?
>
> Thanks,
>
> tglx

Thanks,
Jason