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

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Message-ID: <52df5013-3188-d27c-7ad7-e05cd4912a3d@st.com>
Date:   Thu, 19 Dec 2019 09:14:20 +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()

And calling readl_poll_timeout_atomic() would probably be better :)


On 19/12/2019 10:01 AM, Fabien DESSENNE wrote:
> 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");