Re: [RFC][PATCH v2 -next 2/2] Adding lock operations to kmsg_dump()/pstore_dump()

[Don Zickus <dzickus@redhat.com>]

On Thu, Nov 10, 2011 at 01:50:25PM +0100, Peter Zijlstra wrote:
> On Fri, 2011-10-21 at 17:21 -0400, Seiji Aguchi wrote:
> > +++ b/fs/pstore/platform.c
> > @@ -97,6 +97,17 @@ static void pstore_dump(struct kmsg_dumper *dumper,
> >         else
> >                 why = "Unknown";
> >  
> > +       /*
> > +        * pstore_dump() is called after smp_send_stop() in panic path.
> > +        * So, spin_lock should be bust for avoiding deadlock.
> > +        */
> > +       if (reason == KMSG_DUMP_PANIC)
> > +               spin_lock_init(&psinfo->buf_lock);
> > +
> > +       /*
> > +        * While a cpu is in NMI handler, other cpus may be running.
> > +        * So, trylock should be called so that lockdep checking works.
> > +        */
> 
> Don't be silly, lockdep doesn't cover NMI, in fact you shouldn't use
> locks from NMI context ever.

Heh.  I would normally agree, but in this case we have a piece of hardware
that can be accessed from normal, irq and NMI context.  I still scratch my
head for the best way to handle this.  This approach was sorta of a
bandaid effort to prevent a deadlock in the NMI panic case.

> 
> >         if (in_nmi()) {
> >                 is_locked = spin_trylock(&psinfo->buf_lock);
> >                 if (!is_locked)
> > diff --git a/kernel/printk.c b/kernel/printk.c
> > index 1455a0d..f51f547 100644
> > --- a/kernel/printk.c
> > +++ b/kernel/printk.c
> > @@ -1730,15 +1730,37 @@ void kmsg_dump(enum kmsg_dump_reason reason)
> >         struct kmsg_dumper *dumper;
> >         const char *s1, *s2;
> >         unsigned long l1, l2;
> > -       unsigned long flags;
> > +       unsigned long flags = 0;
> > +       int is_locked = 0;
> >  
> >         /* Theoretically, the log could move on after we do this, but
> >            there's not a lot we can do about that. The new messages
> >            will overwrite the start of what we dump. */
> > -       raw_spin_lock_irqsave(&logbuf_lock, flags);
> > +
> > +       /*
> > +        * kmsg_dump() is called after smp_send_stop() in panic path.
> > +        * So, spin_lock should be bust for avoiding deadlock.
> > +        */
> > +       if (reason == KMSG_DUMP_PANIC)
> > +               raw_spin_lock_init(&logbuf_lock); 
> 
> In both cases where you bust the spinlock at least yell loudly and
> disable lock debugging.
> 
> And I guess this is where Don wants to use NMIs for smp_send_stop() so
> what you get around the fact that this lock you're busting disabled
> IRQs?

:-)  I thought it would be safer to bust spinlocks if we could have a
better gaurantee the other cpus were not accessing the hardware at the
same time.

> 
> All in all this patch is way ugly and doesn't make me feel all warm and
> fuzzy.

I understand.

Cheers,
Don