linux-kernel - Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

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Date:   Wed, 17 Apr 2019 15:58:41 +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>,
        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 v4 11/16] locking/rwsem: Enable readers spinning on writer

On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
> +/*
> + * Try to acquire read lock before the reader is put on wait queue.
> + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
> + * is ongoing.
> + */
> +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
> +{
> +	long count = atomic_long_read(&sem->count);
> +
> +	if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
> +		return false;
> +
> +	count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
> +	if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
> +		rwsem_set_reader_owned(sem);
> +		lockevent_inc(rwsem_opt_rlock);
> +		return true;
> +	}
> +
> +	/* Back out the change */
> +	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> +	return false;
> +}

Doesn't a cmpxchg 'loop' make more sense here?