linux-kernel - Re: [PATCH] dmaengine: pl330: fix double lock

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Date:   Wed, 25 Jan 2017 15:35:57 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     iari@....dk
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        Iago Abal <mail@...oabal.eu>
Subject: Re: [PATCH] dmaengine: pl330: fix double lock

On Wed, Jan 11, 2017 at 02:00:21PM +0100, iari@....dk wrote:
> From: Iago Abal <mail@...oabal.eu>
> 
> The static bug finder EBA (http://www.iagoabal.eu/eba/) reported the
> following double-lock bug:
> 
>     Double lock:
>     1. spin_lock_irqsave(pch->lock, flags) at pl330_free_chan_resources:2236;
>     2. call to function `pl330_release_channel' immediately after;
>     3. call to function `dma_pl330_rqcb' in line 1753;
>     4. spin_lock_irqsave(pch->lock, flags) at dma_pl330_rqcb:1505.
> 
> I have fixed it as suggested by Marek Szyprowski.
> 
> First, I have replaced `pch->lock' with `pl330->lock' in functions
> `pl330_alloc_chan_resources' and `pl330_free_chan_resources'. This avoids
> the double-lock by acquiring a different lock than `dma_pl330_rqcb'.
> 
> NOTE that, as a result, `pl330_free_chan_resources' executes
> `list_splice_tail_init' on `pch->work_list' under lock `pl330->lock',
> whereas in the rest of the code `pch->work_list' is protected by
> `pch->lock'. I don't know if this may cause race conditions. Similarly
> `pch->cyclic' is written by `pl330_alloc_chan_resources' under
> `pl330->lock' but read by `pl330_tx_submit' under `pch->lock'.
> 
> Second, I have removed locking from `pl330_request_channel' and
> `pl330_release_channel' functions. Function `pl330_request_channel' is
> only called from `pl330_alloc_chan_resources', so the lock is already
> held. Function `pl330_release_channel' is called from
> `pl330_free_chan_resources', which already holds the lock, and from
> `pl330_del'. Function `pl330_del' is called in an error path of
> `pl330_probe' and at the end of `pl330_remove', but I assume that there
> cannot be concurrent accesses to the protected data at those points.

Applied, thanks

-- 
~Vinod