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Subject: Re: [PATCH v1 1/8] block: refcount the request_queue early in
 __device_add_disk()
To:     Luis Chamberlain <mcgrof@kernel.org>, axboe@kernel.dk
Cc:     bvanassche@acm.org, ming.lei@redhat.com, hch@infradead.org,
        jack@suse.cz, osandov@fb.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210512064629.13899-1-mcgrof@kernel.org>
 <20210512064629.13899-2-mcgrof@kernel.org>
From:   Hannes Reinecke <hare@suse.de>
Message-ID: <ae439ba5-ad0e-f898-8eda-9c5533af58f8@suse.de>
Date:   Wed, 12 May 2021 17:07:54 +0200
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On 5/12/21 8:46 AM, Luis Chamberlain wrote:
> We refcount the request_queue right now towards the end of the
> __device_add_disk(), however when we add error handling on this
> function we'll want to refcount the request_queue first, to help
> make less complicated changes on drivers on their error paths.
> 
> For instance, today a driver may call add_disk without error handling
> but still handle other errors:
> 
> int foo_block_add(...)
> {
> 	...
> 	queue = blk_mq_init_queue(...);
> 	...
> 	disk->queue = queue;
> 	disk = alloc_disk(...);
> 	if (!disk)
> 		goto out_free_queue;
> 	...
>          add_disk(disk);
> 	...
>          return 0;
> 
> out_free_queue:
>          blk_cleanup_queue(queue);
> 	/* Note: we never call put_disk() as add_disk() never failed */
> 	...
> }
> 
> We want drivers to cleanup with put_disk() on the error path if
> add_disk() fails. However, calling blk_cleanup_queue() will already
> put the queue, and so the last put_disk() on the error path will
> be extra. This can be simplified later if once error handling is
> added to __device_add_disk(), if refcounting the request_queue
> fails right away on __device_add_disk() we just return early and
> set disk->NULL for the driver. That would ensure driver error
> paths chug on with their error paths, and all they'd need to
> expand with is the missing put_disk().
> 
> The collateral evolution for adding error paths for add_disk() becomes
> larger with the alternative of replacing the blk_cleanup_queue() with
> a put_disk(). We'd still need to sprinkle then some blk_cleanup_queue()
> calls on the driver paths up above prior to add_disk(). And how would
> we know we reached a part of add_disk() which did refcount then?
> 
> A related commit is 5a0ec388ef0 ("pktcdvd: Fix pkt_setup_dev() error
> path") which *had* to take the approach of removing the blk_cleanup_queue()
> because otherwise the driver crashes.
> 
> Moving this to the top ensure our future error path can easily just
> handle this itself. For instance, if it was not able to refcount the
> request_queue it can disk->queue to NULL, that way allowing a
> blk_cleanup_queue() call followed but a put_disk(). And if the
> refcount was incremented, we'd still be able to keep the same error
> path of blk_cleanup_queue() followed by put_disk().
> 
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>   block/genhd.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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