Re: __might_sleep in uio_read()?

From: Andy Grover
Date: Thu Sep 03 2015 - 14:47:40 EST

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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