linux-kernel - Re: [PATCH] media: sti: bdisp: fix a possible sleep-in-atomic-context bug in bdisp_device

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Date:   Thu, 19 Dec 2019 09:01:17 +0000
From:   Fabien DESSENNE <fabien.dessenne@...com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>,
        "mchehab@...nel.org" <mchehab@...nel.org>
CC:     "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] media: sti: bdisp: fix a possible sleep-in-atomic-context
 bug in bdisp_device_run()

Hi Jia_Ju,


This is a good finding. See my remarks below.


On 18/12/2019 3:05 PM, Jia-Ju Bai wrote:
> The driver may sleep while holding a spinlock.
> The function call path (from bottom to top) in Linux 4.19 is:
>
> drivers/media/platform/sti/bdisp/bdisp-hw.c, 385:
> 	msleep in bdisp_hw_reset
> drivers/media/platform/sti/bdisp/bdisp-v4l2.c, 341:
> 	bdisp_hw_reset in bdisp_device_run
> drivers/media/platform/sti/bdisp/bdisp-v4l2.c, 317:
> 	_raw_spin_lock_irqsave in bdisp_device_run
>
> To fix this bug, msleep() is replaced with mdelay().
>
> This bug is found by a static analysis tool STCheck written by myself.
>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>   drivers/media/platform/sti/bdisp/bdisp-hw.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/media/platform/sti/bdisp/bdisp-hw.c b/drivers/media/platform/sti/bdisp/bdisp-hw.c
> index 4372abbb5950..1a56348805a2 100644
> --- a/drivers/media/platform/sti/bdisp/bdisp-hw.c
> +++ b/drivers/media/platform/sti/bdisp/bdisp-hw.c
> @@ -382,7 +382,7 @@ int bdisp_hw_reset(struct bdisp_dev *bdisp)
>   	for (i = 0; i < POLL_RST_MAX; i++) {
>   		if (readl(bdisp->regs + BLT_STA1) & BLT_STA1_IDLE)
>   			break;
> -		msleep(POLL_RST_DELAY_MS);
> +		mdelay(POLL_RST_DELAY_MS);

In general, use of mdelay is discouraged. Here we can use udelay instead 
of, with a max value of MAX_UDELAY_MS which is 2 on ARM.

So, instead of running 50 (POLL_RST_MAX) times a 20ms 
(POLL_RST_DELAY_MS) delay, we can run 500 times a 2ms (2000us) delay.

The following changes shall be fine:

     POLL_RST_MAX 50 --> 500

     POLL_RST_DELAY_MS  20 --> 2

     mdelay(POLL_RST_DELAY_MS) --> udelay(POLL_RST_DELAY_MS * 1000)


BR

Fabien


>   	}
>   	if (i == POLL_RST_MAX)
>   		dev_err(bdisp->dev, "Reset timeout\n");