Hi,
Since proctree_lock is a sx lock which uses 2 condition variables,
they alredy drop Giant (DROP_GIANT()) before sleeping. Are you secure
it is the right thing to do here?
Yes I am.
sx_slock() can call cv_wait() which later can call cv_wait_unlock ...
if there is already a lock we need to share ... After a possible
sleep cv_wait_unlock looks like:
cv_wait_unlock(struct cv *cvp, struct mtx *mp)
[...]
sleepq_lock(cvp);
cvp->cv_waiters++;
DROP_GIANT();
mtx_unlock(mp);
[...]
It can call DROP_GIANT for a short amount of time. This was the race.
I already fixed it (but haven't seen the real cause for the crashes) once in
STABLE and HEAD, but backed it out again because I (and jhb) were sure that the
tty driver was protected fully by the GIANT lock after I fixed severall
possible race places in kern_proc.c. I had no idea that sx_slock() and sx_slock
both can drop GIANT for a short time and that we need carefully recheck
variables.
Martin
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