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

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Date:   Mon, 21 Dec 2020 18:13:01 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>, 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 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,

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.

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

Thanks,

Bart.