Re: floppy.c soft lockup

[Mark Hounschell <markh@compro.net>]

Oleg Nesterov wrote:
> 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?
> 

When I kill the main process all its threads also go away. Including the floppy thread.
Nothing notable happens with this kernel. On previous (2.6.18) I would get a dump
from the floppy driver in the syslog when I killed the 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);
> 

The patch does make it work.

Regards
Mark