linux-kernel - Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

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Date:   Tue, 22 Dec 2020 11:15:20 +0000
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>, <axboe@...nel.dk>,
        <ming.lei@...hat.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <hch@....de>, <hare@...e.de>, <ppvk@...eaurora.org>,
        <kashyap.desai@...adcom.com>, <linuxarm@...wei.com>
Subject: Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

On 22/12/2020 02:13, Bart Van Assche wrote:
> On 12/21/20 10:47 AM, John Garry wrote:
>> Yes, I agree, and I'm not sure what I wrote to give that impression.
>>
>> About "root partition", above, I'm just saying that / is mounted on a
>> sda partition:
>>
>> root@...ntu:/home/john# mount | grep sda
>> /dev/sda2 on / type ext4 (rw,relatime,errors=remount-ro,stripe=32)
>> /dev/sda1 on /boot/efi type vfat
>> (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
> Hi John,
> 

Hi Bart, Ming,

> Thanks for the clarification. I want to take back my suggestion about
> adding rcu_read_lock() / rcu_read_unlock() in blk_mq_tagset_busy_iter()
> since it is not allowed to sleep inside an RCU read-side critical
> section, since blk_mq_tagset_busy_iter() is used in request timeout
> handling and since there may be blk_mq_ops.timeout implementations that
> sleep.

Yes, that's why I was going with atomic, rather than some 
synchronization primitive which may sleep.

> 
> Ming's suggestion to serialize blk_mq_tagset_busy_iter() and
> blk_mq_free_rqs() looks interesting to me.
> 

So then we could have something like this:

---8<---

  -435,9 +444,13 @@ void blk_mq_queue_tag_busy_iter(struct request_queue 
*q, busy_iter_fn *fn,
	if (!blk_mq_hw_queue_mapped(hctx))
			continue;

+	while (!atomic_inc_not_zero(&tags->iter_usage_counter));
+
	if (tags->nr_reserved_tags)
		bt_for_each(hctx, tags->breserved_tags, fn, priv, true);
	bt_for_each(hctx, tags->bitmap_tags, fn, priv, false);

+	atomic_dec(&tags->iter_usage_counter);
}

blk_queue_exit(q);

--->8---

And similar for blk_mq_tagset_busy_iter(). How about it?

Thanks,
John