Re: 2.6.33.[56]-rt23: howto create repeatable explosion in wakeup_next_waiter()

[Mike Galbraith <efault@gmx.de>]

On Thu, 2010-07-08 at 19:11 -0700, Darren Hart wrote:

> So the WARN_ON sequence is obviously wrong, if it's critical it should
> be a BUG(), if not we shouldn't dereference what we know to be null. The
> following patch avoids the NULL pointer dereference in the WARN_ON. With
> this patch the NULL WARN_ON makes it to the console, and test runs to
> completion with no obvious negative side effects. I'm only posting for
> reference at this point, as while this may be necessary, it isn't the
> right "solution".

I've been slogging through the locking under the assumption that
pi_blocked_on->task pointing to a stranger is very bad juju, but you're
right, the only obviously evil consequence I see is tripping over the
fallout in WARN_ON().

> Tonight/Tomorrow I'll review the rtmutex and futex code to try and fully
> understand (again) the usage of pi_blocked_on and if we need to avoid
> this scenario, or if we need to handle it "gracefully".

I hope you find it, I'm going blind crawling in endless circles :)

	-Mike