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Date: Mon, 11 Feb 2019 11:57:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Subject: Re: [PATCH] locking/rwsem: Remove arch specific rwsem files
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Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
 Davidlohr Bueso <dave@stgolabs.net>, linux-ia64@vger.kernel.org,
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 Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@redhat.com>,
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 Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Feb 11, 2019 at 10:40:44AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 11, 2019 at 10:36:01AM +0100, Peter Zijlstra wrote:
> > On Sun, Feb 10, 2019 at 09:00:50PM -0500, Waiman Long wrote:
> > > +static inline int __down_read_trylock(struct rw_semaphore *sem)
> > > +{
> > > +	long tmp;
> > > +
> > > +	while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> > > +		if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> > > +				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
> > > +			return 1;
> > 
> > That really wants to be:
> > 
> > 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
> > 						    tmp + RWSEM_ACTIVE_READ_BIAS))
> > 
> > > +		}
> > > +	}
> > > +	return 0;
> > > +}
> 
> Also, the is the one case where LL/SC can actually do 'better'. Do you
> have benchmarks for say PowerPC or ARM64 ?

Ah, I see they already used asm-generic/rwsem.h which has similar code
to the above.