Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 2011-04-21 at 15:02 +0800, Yong Zhang wrote:
> On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> >  ---
> >
> > The above is the case when the unsafe lock is taken while holding
> > a lock taken in irq context. But when a lock is taken that also
> > grabs a unsafe lock, the call chain is shown:
> >
> >  ---
> > other info that might help us debug this:
> >
> > Chain exists of:
> >  &rq->lock --> lockA --> lockC
> >
> >  Possible interrupt unsafe locking scenario:
> >
> >       CPU0                    CPU1
> >       ----                    ----
> >  lock(lockC);
> >                               local_irq_disable();
> >                               lock(&rq->lock);
> >                               lock(lockA);
> >  <Interrupt>
> >    lock(&rq->lock);
> >
> >  *** DEADLOCK ***
> 
> Or we could show this:
> Chain exists of:
> &rq->lock --> lockA --> lockC
> 
>  Possible interrupt unsafe locking scenario:
> 
>       CPU0                    CPU1                           CPU2
>       ----                    ----                                     ----
>  lock(lockC);
>                               local_irq_disable();
>                               lock(&rq->lock);            lock(lockA);
>                               lock(lockA);                   lock(lockC);
>  <Interrupt>
>    lock(&rq->lock);
> 
>  *** DEADLOCK ***


We could but I prefer not to ;) We have some chains that are 8 locks
deep. I really don't want to scatter that entirely across the screen.
Hence my "Chain exists.." statement, following an example that any
kernel developer can (with a little thinking) see is a possible
deadlock.

In fact, this code doesn't even look at the full chain, it only examines
3 locks in the chain, and lets the developer figure out the rest. I
could show the entire chain too.

Thanks,

-- Steve