linux-kernel - Re: floppy.c soft lockup

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Message-ID: <20070531192256.GA88@tv-sign.ru>
Date:	Thu, 31 May 2007 23:22:56 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Mark Hounschell <markh@...pro.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: floppy.c soft lockup

On 05/31, Mark Hounschell wrote:
>
> Oleg Nesterov wrote:
> > On 05/31, Mark Hounschell wrote:
> >>
> >> 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.

Aha, I missed the word "thread", this is the single process.

Still, this means that flush_workqueue() completes when other sub-threads go away,
otherwise the thread doing ioctl() couldn't exit.

Thank you very much.

So, the main question is: is it possible that one of RT processes/threads pins itself
to some CPU and eats 100% cpu power?

>                                            On previous (2.6.18) I would get a dump
> from the floppy driver in the syslog when I killed the process.

Could you send me this output? just in case...

> > --- 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.

I do not understand floppy.c, absolutely, so I am not sure this patch is correct.

Even if correct, this patch doesn't solve this problem (if we really understand
what's going on). cancel_work_sync() may still hang if floppy_work->func() runs
on the starved CPU. This is unlikely, but possible.

Thanks!

Oleg.

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