From: Ming Lei <ming.lei@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@infradead.org>,
Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.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: Fri, 7 May 2021 08:11:07 +0800 [thread overview]
Message-ID: <YJSFm99PUlLedF+D@T590> (raw)
In-Reply-To: <b21d9e07-577b-f9cd-257f-323a82d8d74d@acm.org>
On Thu, May 06, 2021 at 07:51:53AM -0700, Bart Van Assche wrote:
> On 5/6/21 5:18 AM, Christoph Hellwig wrote:
> > 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.
>
> I'm not sure that would be a better name since I don't think that it is
> necessary to hold that lock around the cmpxchg() calls. How about using
> something like the following code in blk_mq_clear_rq_mapping() instead
> of the code in v5 of patch 3/4?
>
> spin_lock_irqsave(&drv_tags->lock, flags);
> spin_unlock_irqrestore(&drv_tags->lock, flags);
>
> list_for_each_entry(page, &tags->page_list, lru) {
> /* use cmpxchg() to clear request pointer selectively */
> }
This way won't work as expected because iterating may happen between
releasing drv_tags->lock and cmpxchg(->rqs[]), then freed requests
may still be touched during iteration after they are freed in blk_mq_free_rqs().
Thanks,
Ming
next prev parent reply other threads:[~2021-05-07 0:11 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-05 14:58 [PATCH V5 0/4] blk-mq: fix request UAF related with iterating over tagset requests Ming Lei
2021-05-05 14:58 ` [PATCH V5 1/4] block: avoid double io accounting for flush request Ming Lei
2021-05-06 6:44 ` Christoph Hellwig
2021-05-05 14:58 ` [PATCH V5 2/4] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter Ming Lei
2021-05-06 6:54 ` Christoph Hellwig
2021-05-06 7:30 ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool Ming Lei
2021-05-06 7:12 ` Christoph Hellwig
2021-05-06 7:34 ` Ming Lei
2021-05-06 12:18 ` Christoph Hellwig
2021-05-06 13:38 ` Ming Lei
2021-05-07 6:57 ` Christoph Hellwig
2021-05-07 7:30 ` Ming Lei
2021-05-06 14:51 ` Bart Van Assche
2021-05-07 0:11 ` Ming Lei [this message]
2021-05-07 1:10 ` Bart Van Assche
2021-05-07 2:05 ` Ming Lei
2021-05-07 3:16 ` Bart Van Assche
2021-05-07 6:31 ` Ming Lei
2021-05-07 6:52 ` Christoph Hellwig
2021-05-08 2:02 ` Bart Van Assche
2021-05-06 15:02 ` Bart Van Assche
2021-05-07 0:13 ` Ming Lei
2021-05-07 1:11 ` Bart Van Assche
2021-05-07 2:06 ` Ming Lei
2021-05-05 14:58 ` [PATCH V5 4/4] blk-mq: clearing flush request reference in tags->rqs[] Ming Lei
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