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

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Message-ID: <1401937739.6680.5.camel@j-VirtualBox>
Date:	Wed, 04 Jun 2014 20:08:59 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	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, Waiman.Long@...com, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

On Wed, 2014-06-04 at 18:10 -0700, Davidlohr Bueso wrote:
> 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.

Yup, I was also initially wondering why we have a slowpath for trylock,
which prompted me to take a look into this function. I'll add some more
information about this to the changelog.

Thanks,
Jason

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