linux-kernel - Re: __might_sleep in uio

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Date:	Thu, 3 Sep 2015 14:26:16 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Andy Grover <agrover@...hat.com>
Cc:	hjk@...sjkoch.de, Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: __might_sleep in uio_read()?

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 {
-- 
Michal Hocko
SUSE Labs
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