Re: BUG: ib_mad ftrace event unsupported migration

[Jason Gunthorpe]

On Wed, Nov 02, 2022 at 02:02:49PM +0000, Leonid Ravich wrote:
> > > > before starting throwing some patch into the the air  I would like to align with you the approach we should take here. 
> > > > 
> > > > my suggestion here : 
> > > >- ftrace infra should verify no migration happen  (end and start happens on same CPU)  in case not we will  throw warning for the issue  .
> > >
> > >The scheduler should have. On entering the ring buffer code
> > >ring_buffer_lock_reserver() it disables preemption and does not
> > >re-enable it until ring_buffer_unlock_commit().
> > >
> > >The only way to migrate is if you re-enable preemption. WHICH IS A
> > >BUG!
> 
> >So what on earth did that?
> 
> >I'm guessing some driver's query_pkey op, but AFAIK we don't have any
> >explicit pre-emption reenablements in the code - unless it is sneaky..
> trace infra uses preempt_disable_notrace/preempt_enable_notrace  to disable/enable preemtion but  my kernel compiled without CONFIG_PREEMPTION so this functions are only barriers - looks like the idea behind was to avoid involuntary preemtion  but in our case it is a voluntary (there is a wait_for_completion in the query_pkey  rabbit hole).

So this tracepoint is just wrong, you can't call a sleepable function
from a tracepoint like that?

Presumably lockdep would/should warn about this?

Delete the pkey logging from the tracepoint, it can't work, I guess.

Jason