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Date:   Wed, 26 Jan 2022 06:21:59 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Jan Kara <jack@suse.cz>, linux-block@vger.kernel.org,
        Ming Lei <ming.lei@redhat.com>
Subject: Re: [PATCH v3 1/5] task_work: export task_work_add()
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On Wed, Jan 26, 2022 at 08:47:17AM +0900, Tetsuo Handa wrote:
> > As far as I can tell we do not need the freeze at all for given that
> > by the time release is called I/O is quiesced.
> 
> Why? lo_release() is called when close() is called. But (periodically-scheduled
> or triggered-on-demand) writeback of previously executed buffered write() calls
> can start while lo_release() or __loop_clr_fd() is running. Then, why not to
> wait for I/O requests to complete?

Let's refine my wording, the above should be "... by the time the final
lo_release is called".  blkdev_put_whole ensures all writeback has finished
and all buffers are gone by writing all data back and waiting for it and then
truncating the pages from blkdev_flush_mapping.

> Isn't that the reason of
> 
> 	} else if (lo->lo_state == Lo_bound) {
> 		/*
> 		 * Otherwise keep thread (if running) and config,
> 		 * but flush possible ongoing bios in thread.
> 		 */
> 		blk_mq_freeze_queue(lo->lo_queue);
> 		blk_mq_unfreeze_queue(lo->lo_queue);
> 	}
> 
> path in lo_release() being there?

This looks completely spurious to me.  Adding Ming who added it.