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From: Bart Van Assche <bvanassche@acm.org>
To: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@lst.de>
Cc: linux-block@vger.kernel.org, Yi Zhang <yi.zhang@redhat.com>
Subject: Re: [PATCH 2/4] block: move wbt allocation into blk_alloc_queue
Date: Thu, 3 Jun 2021 17:44:00 -0700	[thread overview]
Message-ID: <35a0f0b7-ad44-26cb-7fb7-d4f56241ff62@acm.org> (raw)
In-Reply-To: <20210525080442.1896417-3-ming.lei@redhat.com>

On 5/25/21 1:04 AM, Ming Lei wrote:
> wbt_init() calls wbt_alloc() which adds allocated wbt instance into
> q->rq_qos. This way is very dangerous because q->rq_qos is accessed in
> IO fast path.
> 
> So far wbt_init() is called in the two code paths:
> 
> 1) blk_register_queue(), when the block device has been exposed to
> usespace, so IO may come when adding wbt into q->rq_qos
> 
> 2) sysfs attribute store, in which normal IO is definitely allowed
> 
> Move wbt allocation into blk_alloc_queue() for avoiding to add wbt
> instance dynamically to q->rq_qos. And before calling wbt_init(), the
> wbt is disabled, so functionally it works as expected.

I don't like this change since it is not generic - it only helps the WBT
implementation.

All rq-qos policies call rq_qos_add() and all these policies take effect
before rq_qos_add() returns. Does the q->rq_qos list perhaps have to be
protected with RCU? Would that be sufficient to fix the crashes reported
in the cover letter?

Thanks,

Bart.

  reply	other threads:[~2021-06-04  0:44 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-25  8:04 [PATCH 0/4] block: fix race between adding wbt and normal IO Ming Lei
2021-05-25  8:04 ` [PATCH 1/4] block: split wbt_init() into two parts Ming Lei
2021-06-04  0:32   ` Bart Van Assche
2021-05-25  8:04 ` [PATCH 2/4] block: move wbt allocation into blk_alloc_queue Ming Lei
2021-06-04  0:44   ` Bart Van Assche [this message]
2021-06-04  1:22     ` Ming Lei
2021-05-25  8:04 ` [PATCH 3/4] block: reuse wbt_set_min_lat for setting wbt->min_lat_nsec Ming Lei
2021-05-25  8:04 ` [PATCH 4/4] block: mark queue init done at the end of blk_register_queue Ming Lei
2021-06-04  0:03 ` [PATCH 0/4] block: fix race between adding wbt and normal IO Ming Lei

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