Re: Stop scheduler on panic

On Thu, Nov 17, 2011 at 10:40:58AM +0200, Alexander Motin wrote:
> On 11/17/11 10:15, Kostik Belousov wrote:
> > On Thu, Nov 17, 2011 at 01:07:38AM +0200, Alexander Motin wrote:
> >> On 17.11.2011 00:21, Andriy Gapon wrote:
> >>> on 16/11/2011 21:27 Fabian Keil said the following:
> >>>> Kostik Belousov<kostikbel_at_gmail.com>  wrote:
> >>>>
> >>>>> I was tricked into finishing the work by Andrey Gapon, who developed
> >>>>> the patch to reliably stop other processors on panic.  The patch
> >>>>> greatly improves the chances of getting dump on panic on SMP host.
> >>>>
> >>>> I tested the patch trying to get a dump (from the debugger) for
> >>>> kern/162036, which currently results in the double fault reported in:
> >>>> http://lists.freebsd.org/pipermail/freebsd-current/2011-September/027766.html
> >>>>
> >>>> It didn't help, but also didn't make anything worse.
> >>>>
> >>>> Fabian
> >>>
> >>> The mi_switch recursion looks very familiar to me:
> >>> mi_switch() at mi_switch+0x270
> >>> critical_exit() at critical_exit+0x9b
> >>> spinlock_exit() at spinlock_exit+0x17
> >>> mi_switch() at mi_switch+0x275
> >>> critical_exit() at critical_exit+0x9b
> >>> spinlock_exit() at spinlock_exit+0x17
> >>> [several pages of the previous three lines skipped]
> >>> mi_switch() at mi_switch+0x275
> >>> critical_exit() at critical_exit+0x9b
> >>> spinlock_exit() at spinlock_exit+0x17
> >>> intr_even_schedule_thread() at intr_event_schedule_thread+0xbb
> >>> ahci_end_transaction() at ahci_end_transaction+0x398
> >>> ahci_ch_intr() at ahci_ch_intr+0x2b5
> >>> ahcipoll() at ahcipoll+0x15
> >>> xpt_polled_action() at xpt_polled_action+0xf7
> >>>
> >>> In fact I once discussed with jhb this recursion triggered from a different
> >>> place.  To quote myself:
> >>> <avg>    spinlock_exit ->  critical_exit ->  mi_switch ->  kdb_switch ->
> >>> thread_unlock ->  spinlock_exit ->  critical_exit ->  mi_switch ->  ...
> >>> <avg>    in the kdb context
> >>> <avg>    this issue seems to be triggered by td_owepreempt being true at 
> >>> the time
> >>> kdb is entered
> >>> <avg>    and there of course has to be an initial spinlock_exit call 
> >>> somewhere
> >>> <avg>    in my case it's because of usb keyboard
> >>> <avg>    I wonder if it would make sense to clear td_owepreempt right 
> >>> before
> >>> calling kdb_switch in mi_switch
> >>> <avg>    instead of in sched_switch()
> >>> <avg>    clearing td_owepreempt seems like a scheduler-independent 
> >>> operation to me
> >>> <avg>    or is it better to just skip locking in usb when kdb_active is set
> >>> <avg>    ?
> >>>
> >>> The workaround described above should work in this case.
> >>> Another possibility is to pessimize mtx_unlock_spin() implementations to 
> >>> check
> >>> SCHEDULER_STOPPED() and to bypass any further actions in that case.  But 
> >>> that
> >>> would add unnecessary overhead to the sunny day code paths.
> >>>
> >>> Going further up the stack one can come up with the following proposals:
> >>> - check SCHEDULER_STOPPED() swi_sched() and return early
> >>> - do not call swi_sched() from xpt_done() if we somehow know that we are 
> >>> in a
> >>> polling mode
> >>
> >> There is no flag in CAM now to indicate polling mode, but if needed, it 
> >> should not be difficult to add one and not call swi_sched().
> > 
> > I have the following change for eons on my test boxes. Without it,
> > I simply cannot get _any_ dump.
> > 
> > diff --git a/sys/cam/cam_xpt.c b/sys/cam/cam_xpt.c
> > index 10b89c7..a38e42f 100644
> > --- a/sys/cam/cam_xpt.c
> > +++ b/sys/cam/cam_xpt.c
> > _at__at_ -4230,7 +4230,7 _at__at_ xpt_done(union ccb *done_ccb)
> >  			TAILQ_INSERT_TAIL(&cam_simq, sim, links);
> >  			mtx_unlock(&cam_simq_lock);
> >  			sim->flags |= CAM_SIM_ON_DONEQ;
> > -			if (first)
> > +			if (first && panicstr == NULL)
> >  				swi_sched(cambio_ih, 0);
> >  		}
> >  	}
> 
> That should be OK for kernel dumping. I was thinking about CAM abusing
> polling not only for dumping. But looking on cases where it does it now,
> may be it is better to rewrite them instead.

So, should I interpret your response as 'Reviewed by" ?