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

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Message-ID: <1401920011.2232.49.camel@j-VirtualBox>
Date:	Wed, 04 Jun 2014 15:13:31 -0700
From:	Jason Low <jason.low2@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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


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