linux-kernel - bi_end_io() is called with lock held: bug or feature?

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Message-ID: <CA+X1aCRtR8gdLj5ekUL3_gBeULEPMedjNyNreABEmoCnceFV0Q@mail.gmail.com>
Date:	Mon, 4 Jul 2011 19:56:51 +0400
From:	Maxim Patlasov <maxim.patlasov@...il.com>
To:	axboe@...nel.dk
Cc:	linux-kernel@...r.kernel.org
Subject: bi_end_io() is called with lock held: bug or feature?

Hi Jens,

While experimenting with some third-party driver, I noticed that
sometimes (at least for empty flush requests) bi_end_io callback is
called with IRQs locally disabled. The cause turned out to be in
blk_end_bidi_request():

>	spin_lock_irqsave(q->queue_lock, flags);
>	blk_finish_request(rq, error);
>	spin_unlock_irqrestore(q->queue_lock, flags);

A relevant part of stack trace is:

> bio_endio+0x18/0x30
> dec_pending+0x172/0x2b0
> clone_endio+0x99/0xd0
> bio_endio+0x18/0x30
> req_bio_endio+0x83/0xc0
> blk_update_request+0xff/0x470
> blk_update_bidi_request+0x22/0xa0
> __blk_end_bidi_request+0x1c/0x40
> __blk_end_request_all+0x1a/0x30
> blk_flush_complete_seq+0x258/0x280
> flush_end_io+0xe5/0x170
> blk_finish_request+0x86/0x280
> blk_end_bidi_request+0x4f/0x80

At first glance it looks a bit weird that in most cases bi_end_io
callback is called w/o any locks held but under some conditions (e.g.
completing empty REQ_FLUSH bio) - with q->queue_lock acquired. Is it a
bug or expected behaviour?

Thanks in advance,
Maxim
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