linux-kernel - Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

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Message-ID: <20190211105729.GH32534@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Feb 2019 11:57:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

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.