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Subject: Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for
 tagset in elevator_exit()
To:     John Garry <john.garry@huawei.com>, Ming Lei <ming.lei@redhat.com>
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From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <e0906b2e-6a2b-ce34-84a1-36eaddbb824d@acm.org>
Date:   Tue, 16 Mar 2021 12:59:07 -0700
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On 3/16/21 10:43 AM, John Garry wrote:
> On 16/03/2021 17:00, Bart Van Assche wrote:
>> I agree that Jens asked at the end of 2018 not to touch the fast path
>> to fix this use-after-free (maybe that request has been repeated more
>> recently). If Jens or anyone else feels strongly about not clearing
>> hctx->tags->rqs[rq->tag] from the fast path then I will make that change. 
> 
> Is that possible for this same approach? I need to check the code more..

If the fast path should not be modified, I'm considering to borrow patch
1/3 from your patch series and to add an rcu_barrier() between the code
that clears the request pointers and that frees the scheduler requests.

> And don't we still have the problem that some iter callbacks may
> sleep/block, which is not allowed in an RCU read-side critical section?

Thanks for having brought this up. Since none of the functions that
iterate over requests should be called from the hot path of a block
driver, I think that we can use srcu_read_(un|)lock() inside bt_iter()
and bt_tags_iter() instead of rcu_read_(un|)lock().

Bart.