cvs commit: src/sys/kern kern_event.c src/sys/sys eventvar.h

[Doug Rabson]

On Wednesday 14 July 2004 19:56, Robert Watson wrote:
> On Wed, 14 Jul 2004, Alfred Perlstein wrote:
> > I'm sure that was a fun panic to hit. :)
>
> Thrilling :-).  Actually, I didn't hit the panic, I merely reasoned
> that it might exist and then read more kqueue code than I've read in
> a while
>
> :-).
> :
> > I can fix this by setting a "sigio in progress" on the kqeue and
> > not calling pgsigio() while one is in progress.
>
> My worry is the inter-subsystem calling.  We often call KNOTE() while
> holding existing locks in the calling subsystem that we can't drop.
> Generally, kqueue is a leaf node subsystem in that it's called from
> many places under many circumstances, and needs to not disrupt the
> calling state by doing "weird things".  What's there before your
> change is not too disruptive/weird; afterwards, we call into the body
> of the process signalling code which requires additional process
> locks.  Note that there are other paths to the same suffering: if any
> other signal is delivered to a process that's monitoring for signals
> with kqueue causing a sigio, you're still recursing into the signal
> subsystem.

Seems to me that the best thing to do is to defer the psigio() to a 
taskqueue that will run in a simpler locking environment.