linux-kernel - Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative

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Date:   Thu, 18 Apr 2019 15:51:51 +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 14/16] locking/rwsem: Guard against making count
 negative

On Sat, Apr 13, 2019 at 01:22:57PM -0400, Waiman Long wrote:
>  inline void __down_read(struct rw_semaphore *sem)
>  {
> +	long count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
> +						   &sem->count);
> +
> +	if (unlikely(count & RWSEM_READ_FAILED_MASK)) {
> +		rwsem_down_read_failed(sem, count);
>  		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
>  	} else {
>  		rwsem_set_reader_owned(sem);

*groan*, that is not provably correct. It is entirely possible to get
enough fetch_add()s piled on top of one another to overflow regardless.

Unlikely, yes, impossible, no.

This makes me nervious as heck, I really don't want to ever have to
debug something like that :-(