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References: <1339596420-18127-1-git-send-email-javi.merino@arm.com>
Date: Wed, 13 Jun 2012 20:43:05 +0530
Message-ID: <CAJe_Zhfv2WdQYyQZE9KrFYHQajmBh31jQk7N26BtYSL20TaCyQ@mail.gmail.com>
Subject: Re: [PATCH] DMA: PL330: Fix racy mutex unlock
From: Jassi Brar <jaswinder.singh@linaro.org>
To: Javi Merino <javi.merino@arm.com>
Cc: linux-kernel@vger.kernel.org, dan.j.williams@intel.com,
        vinod.koul@intel.com
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On 13 June 2012 19:37, Javi Merino <javi.merino@arm.com> wrote:
> pl330_update() stores a pointer to the thrd->req that finished, which
> contains a pointer to the corresponding pl330_req. ?This is done with
> the pl330_lock held. ?Then, it iterates through the req_done list,
> calling the callback for each of the requests that are done. ?The
> problem is that the driver releases the lock before calling the
> callback for each of the callbacks. ?pl330_submit_req() running in
> another processor can then acquire the lock and insert another request
> in one of the thrd->req that hasn't been processed yet, replacing the
> pointer to pl330_req there. ?When the callback returns in
> pl330_update() and the next rqdone is popped from the list, it
> dereferences the pl330_req pointer to the just scheduled pl330_req,
> instead of the one that has finished, calling pl330 with the wrong r.
>
> This patch fixes this by storing the pointer to pl330_req directly in
> the list.
>
.....
> @@ -1683,7 +1683,7 @@ static void pl330_dotask(unsigned long data)
> ?/* Returns 1 if state was updated, 0 otherwise */
> ?static int pl330_update(const struct pl330_info *pi)
> ?{
> - ? ? ? struct _pl330_req *rqdone;
> + ? ? ? struct pl330_req *rqdone, *tmp;
> ? ? ? ?struct pl330_dmac *pl330;
> ? ? ? ?unsigned long flags;
> ? ? ? ?void __iomem *regs;
> @@ -1750,7 +1750,10 @@ static int pl330_update(const struct pl330_info *pi)
> ? ? ? ? ? ? ? ? ? ? ? ?if (active == -1) /* Aborted */
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?continue;
>
> - ? ? ? ? ? ? ? ? ? ? ? rqdone = &thrd->req[active];
> + ? ? ? ? ? ? ? ? ? ? ? /* Detach the req */
> + ? ? ? ? ? ? ? ? ? ? ? rqdone = thrd->req[active].r;
> + ? ? ? ? ? ? ? ? ? ? ? thrd->req[active].r = NULL;
> +
Doesn't this movement of "Detach the req" chunk effectively remain the
same? Since that was already protected by the same lock. I thought I
deliberately took care of that already.

Do you see some real problem fixed by this patch? Info about that
could help me better understand if I missed something here.

Thanks.
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