linux-kernel - Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

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Message-ID: <20190417081739.GB4038@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Apr 2019 10:17:39 +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 07/16] locking/rwsem: Implement lock handoff to
 prevent lock starvation

On Sat, Apr 13, 2019 at 01:22:50PM -0400, Waiman Long wrote:
> +/*
> + * This is safe to be called without holding the wait_lock.

Because.... @waiter is *our* waiter and it's not going anywhere. So when
it's first, it stays first until we do something about it.

> + */
> +static inline bool
> +rwsem_waiter_is_first(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
> +{
> +	return list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
> +			== waiter;
> +}