linux-kernel - Re: __might_sleep in uio

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Date:	Thu, 3 Sep 2015 11:47:34 -0700
From:	Andy Grover <agrover@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	hjk@...sjkoch.de, Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: __might_sleep in uio_read()?

On 09/03/2015 05:26 AM, Michal Hocko wrote:
> On Wed 02-09-15 15:45:10, Andy Grover wrote:
>> Hi Hans and Greg,
>>
>> Is this an issue with uio? I swear it didn't used to throw this warning...
>>
>> Thanks -- Andy
>>
>> [ 5174.883261] ------------[ cut here ]------------
>> [ 5174.883617] WARNING: CPU: 0 PID: 1532 at
>> /home/agrover/git/kernel/kernel/sched/core.c:7389 __might_sleep+0x7d/0x90()
>> [ 5174.884407] do not call blocking ops when !TASK_RUNNING; state=1 set at
>> [<ffffffffa02a5821>] uio_read+0x91/0x170 [uio]
>
> The warning says that the driver is calling copy_to_user with
> TASK_INTERRUPTIBLE which is wrong in general because this context can
> sleep and a schedule would destroy the state. It doesn't matter here
> because the code would break out from the loop regardless of the
> copy_to_user return value.
>
> I assume that TASK_INTERRUPTIBLE is necessary before the event_count
> check to prevent from wake up races. If that is the case then you can
> simply do:
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index 3257d4220d01..7d8959e3833b 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -524,6 +524,7 @@ static ssize_t uio_read(struct file *filep, char __user *buf,
>
>   		event_count = atomic_read(&idev->event);
>   		if (event_count != listener->event_count) {
> +			__set_current_state(TASK_RUNNING);
>   			if (copy_to_user(buf, &event_count, count))
>   				retval = -EFAULT;
>   			else {
>

This certainly makes the warning go away. If this looks good to everyone 
else can we get this change in?

Thanks -- Andy

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