Re: floppy.c soft lockup

Oleg Nesterov
2007-05-31T17:06:21Z

On 05/31, Mark Hounschell wrote:
>
> Andrew Morton wrote:
> > On Tue, 29 May 2007 13:31:05 -0400 Mark Hounschell <markh@compro.net> wrote:
> >
> >> Changes in floppy.c from 2.6.17 and 2.6.18 have broken an application I have. I have tracked
> >> it down to a single line of code. When the following patch is applied to the version in 2.6.18
> >> my application works.
> >>
> >> --- linux-2.6.18/drivers/block/floppy.c 2006-09-19 23:42:06.000000000 -0400
> >> +++ linux-2.6.18-crt/drivers/block/floppy.c 2007-05-29 09:12:20.000000000 -0400
> >> @@ -893,7 +893,6 @@
> >> set_current_state(TASK_RUNNING);
> >> remove_wait_queue(&fdc_wait, &wait);
> >>
> >> - flush_scheduled_work();
> >> }
> >> command_status = FD_COMMAND_NONE;
> >>
> > Interesting. I'd expect that the calling process is spinning, with realtime
> > policy and is expecting some other process to do something (ie: run a workqueue).
> >
> > If you keep the process and irq affinities, and disable the realtime policy
> > does that also prevent the problem?
> >
>
> Yes it does.
>
> > It would be interesting it you could capture a few task traces while it is stuck:
> > echo 1 > /proc/sys/kernel/sysrq then do ALT-SYSRQ-P a bunch of times and ALT-SYSRQ-T,
> > see if you can work out where the CPU is stuck.
> >
>
> I've attached the syslog output as a result of doing the above. I can't really make any kind of
> determination from it myself as I don't really knowing what I'm looking at.

Could you show the full output? There are no events/* or process doing ioctl()
in sysrq.txt you attached.

> > ALso, 2.6.22-rc3 might have accidentally fixed this.
> >
>
> No. Same thing there. The traces attached are using 2.6.22-rc3.
>
> Basically the main RT-process (which is a CPU bound process on processor-2) signals a
> thread to do some I/O. That RT-thread (running on the other processor) does a simple

If the main RT-process monopolizes processor-2, flush_workqueue() (or cancel_work_sync())
can hang of course, we can do nothing.

> ioctl(Q->DevSpec1, FDSETPRM, &medprm)
>
> and there is no return from the call. That thread is hung.

What happens if you kill the main RT-process?

Could you try the patch below? Just to see if it makes any difference.

Oleg.

(against 2.6.22-rcX)

--- OLD/drivers/block/floppy.c~ 2007-04-03 13:04:58.000000000 +0400
+++ OLD/drivers/block/floppy.c 2007-05-31 20:50:18.000000000 +0400
@@ -862,6 +862,8 @@ static void set_fdc(int drive)
FDCS->reset = 1;
}

+static DECLARE_WORK(floppy_work, NULL);
+
/* locks the driver */
static int _lock_fdc(int drive, int interruptible, int line)
{
@@ -893,7 +895,7 @@ static int _lock_fdc(int drive, int inte
set_current_state(TASK_RUNNING);
remove_wait_queue(&fdc_wait, &wait);

- flush_scheduled_work();
+ cancel_work_sync(&floppy_work);
}
command_status = FD_COMMAND_NONE;

@@ -992,8 +994,6 @@ static void empty(void)
{
}

-static DECLARE_WORK(floppy_work, NULL);
-
static void schedule_bh(void (*handler) (void))
{
PREPARE_WORK(&floppy_work, (work_func_t)handler);