linux-kernel - Re: [PATCH] DMA: PL330: Fix racy mutex unlock

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Date:	Wed, 13 Jun 2012 20:43:05 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	Javi Merino <javi.merino@....com>
Cc:	linux-kernel@...r.kernel.org, dan.j.williams@...el.com,
	vinod.koul@...el.com
Subject: Re: [PATCH] DMA: PL330: Fix racy mutex unlock

On 13 June 2012 19:37, Javi Merino <javi.merino@....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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