Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 18 Apr 2019 15:51:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
 negative
Message-ID: <20190418135151.GB12232@hirez.programming.kicks-ass.net>
References: <20190413172259.2740-1-longman@redhat.com>
 <20190413172259.2740-15-longman@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190413172259.2740-15-longman@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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 :-(