Re: [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool

From: Christoph Hellwig <hch@infradead.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.de>,
	John Garry <john.garry@huawei.com>,
	David Jeffery <djeffery@redhat.com>
Subject: Re: [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool
Date: Thu, 6 May 2021 13:18:49 +0100	[thread overview]
Message-ID: <20210506121849.GA400362@infradead.org> (raw)
In-Reply-To: <YJOb+a85Cpb56Mdz@T590>

On Thu, May 06, 2021 at 03:34:17PM +0800, Ming Lei wrote:
> > {
> > 	struct blk_mq_tags *drv_tags = set->tags[hctx_idx];
> > 	unsigned int i = 0;
> > 	unsigned long flags;
> > 
> > 	spin_lock_irqsave(&drv_tags->lock, flags);
> > 	for (i = 0; i < set->queue_depth; i++)
> > 		WARN_ON_ONCE(refcount_read(&drv_tags->rqs[i]->ref) != 0);
> > 		drv_tags->rqs[i] = NULL;
> 
> drv_tags->rqs[] may be assigned from another LUN at the same time, then
> the simple clearing will overwrite the active assignment. We just need
> to clear mapping iff the stored rq will be freed.

So.  Even a different LUN shares the same tagset.  So I can see the
need for the cmpxchg (please document it!), but I don't see the need
for the complex iteration.  All the rqs are freed in one single loop,
so we can just iterate through them sequentially.

Btw, I think ->lock might better be named ->clear_lock to document its
purpose better.