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Date: Mon, 11 Feb 2019 10:40:44 +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,
 Tim Chen <tim.c.chen@linux.intel.com>, Arnd Bergmann <arnd@arndb.de>,
 linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org, x86@kernel.org,
 Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
 Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
 linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
 Thomas Gleixner <tglx@linutronix.de>, linuxppc-dev@lists.ozlabs.org,
 Andrew Morton <akpm@linux-foundation.org>,
 linux-arm-kernel@lists.infradead.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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 ?