linux-kernel - Re: [PATCH v8 16/19] locking/rwsem: Guard against making count negative

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Date:   Tue, 11 Jun 2019 15:13:59 +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 16/19] locking/rwsem: Guard against making count
 negative

On Mon, May 20, 2019 at 04:59:15PM -0400, Waiman Long wrote:
>  static struct rw_semaphore __sched *
> +rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long adjustment)
>  {
> +	long count;
>  	bool wake = false;
>  	struct rwsem_waiter waiter;
>  	DEFINE_WAKE_Q(wake_q);
>  
> +	if (unlikely(!adjustment)) {
> +		/*
> +		 * This shouldn't happen. If it does, there is probably
> +		 * something wrong in the system.
> +		 */
> +		WARN_ON_ONCE(1);

	if (WARN_ON_ONCE(!adjustment)) {

> +
> +		/*
> +		 * An adjustment of 0 means that there are too many readers
> +		 * holding or trying to acquire the lock. So disable
> +		 * optimistic spinning and go directly into the wait list.
> +		 */
> +		if (rwsem_test_oflags(sem, RWSEM_RD_NONSPINNABLE))
> +			rwsem_set_nonspinnable(sem);

ISTR rwsem_set_nonspinnable() already does that test, so no need to do
it again, right?

> +		goto queue;
> +	}
> +
>  	/*
>  	 * Save the current read-owner of rwsem, if available, and the
>  	 * reader nonspinnable bit.