linux-kernel - Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

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Date:   Tue, 4 Jun 2019 12:58:48 +0200
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>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning

On Mon, May 20, 2019 at 04:59:14PM -0400, Waiman Long wrote:
> +static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
> +					      long last_rowner)
> +{
> +	long owner = atomic_long_read(&sem->owner);
> +
> +	if (!(owner & RWSEM_READER_OWNED))
> +		return false;
> +
> +	owner	    &= ~RWSEM_OWNER_FLAGS_MASK;
> +	last_rowner &= ~RWSEM_OWNER_FLAGS_MASK;
> +	if ((owner != last_rowner) && rwsem_try_read_lock_unqueued(sem)) {

just because I'm struggling with sleep deprivation and the big picture
isn't making sense,.. you can write that like:

	((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK)

> +		lockevent_inc(rwsem_opt_rlock2);
> +		lockevent_add(rwsem_opt_fail, -1);
> +		return true;
> +	}
> +	return false;
> +}