linux-kernel - Re: __might_sleep in uio

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Date:	Thu, 3 Sep 2015 13:12:28 -0700
From:	Andy Grover <agrover@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Michal Hocko <mhocko@...nel.org>, hjk@...sjkoch.de,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: __might_sleep in uio_read()?

On 09/03/2015 12:12 PM, Greg KH wrote:
> On Thu, Sep 03, 2015 at 11:47:34AM -0700, Andy Grover wrote:
>> 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?
>
> What changed to require this?  Why is this suddenly showing up now?

I'm working on drivers/target/target_core_user.c, a SCSI userspace 
passthrough that was added in 3.18, aka TCMU, which uses uio.

The checks for !TASK_RUNNING were added in 3.19 (8eb23b9f3 and 00845eb96)

...and I'm just getting back to TCMU development after a bit, so maybe 
I'm the first one to call uio_read since those checks were added in 3.19?

-- Andy

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