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

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Message-ID: <alpine.DEB.2.10.1406042337110.18296@nanos>
Date:	Wed, 4 Jun 2014 23:54:43 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jason Low <jason.low2@...com>
cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	peter@...leysoftware.com, riel@...hat.com, hpa@...or.com,
	walken@...gle.com, davidlohr@...com, Waiman.Long@...com,
	aswin@...com, scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it
 is unlocked

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.

> 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?

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
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