linux-kernel - Re: __might_sleep in uio

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Date:	Thu, 3 Sep 2015 20:59:19 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Andy Grover <agrover@...hat.com>
Cc:	Michal Hocko <mhocko@...nel.org>, hjk@...sjkoch.de,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: __might_sleep in uio_read()?

On Thu, Sep 03, 2015 at 01:12:28PM -0700, Andy Grover wrote:
> 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?

If so, that's sad.

greg k-h
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