You can use dwarf4 if you use GDB from ports ---- On Wed, 15 Jun
2016 04:50:00 -0700 Peter Holm<pe...@holm.cc> wrote ----On Wed, Jun 15, 2016
at 11:11:43AM +0300, Konstantin Belousov wrote: > On Tue, Jun 14, 2016 at
10:26:14PM -0500, Eric Badger wrote: > > I believe they all have more or less
the same cause. The crashes occur > > because we acquire a knlist lock via the
KN_LIST_LOCK macro, but when we > > call KN_LIST_UNLOCK, the knote???s knlist
reference (kn->kn_knlist) has > > been cleared by another thread. Thus we are
unable to unlock the > > previously acquired lock and hold it until something
causes us to crash > > (such as the witness code noticing that we???re
returning to userland with > > the lock still held). > ... > > I believe
there???s also a small window where the KN_LIST_LOCK macro > > checks
kn->kn_knlist and finds it to be non-NULL, but by the time it > > actually
dereferences it, it has become NULL. This would produce the > > ???page fault
while in kernel mode??? crash. > > > > If someone familiar with this code sees
an obvious fix, I???ll be happy to > > test it. Otherwise, I???d appreciate
any advice on fixing this. My first > > thought is that a ???struct knote???
ought to have its own mutex for > > controlling access to the flag fields and
ideally the ???kn_knlist??? field. > > I.e., you would first acquire a
knote???s lock and then the knlist lock, > > thus ensuring that no one could
clear the kn_knlist variable while you > > hold the knlist lock. The knlist
lock, however, usually comes from > > whichever event producing entity the
knote tracks, so getting lock > > ordering right between the per-knote mutex
and this other lock seems > > potentially hard. (Sometimes we call into
functions in kern_event.c with > > the knlist lock already held, having been
acquired in code outside of > > kern_event.c. Consider, for example, calling
KNOTE_LOCKED from > > kern_exit.c; the PROC_LOCK macro has already been used
to acquire the > > process lock, also serving as the knlist lock). > This
sounds as a good and correct analysis. I tried your test program > for around a
hour on 8-threads machine, but was not able to trigger the > issue. Might be
Peter have better luck reproducing them. Still, I think > that the problem is
there. > > IMO we should simply avoid clearing kn_knlist in knlist_remove().
The > member is only used to get the locking function pointers, otherwise >
code relies on KN_DETACHED flag to detect on-knlist condition. See > the patch
below. > > > > > Apropos of the knlist lock and its provenance: why is a lock
from the > > event producing entity used to control access to the knlist and
knote? > > Is it generally desirable to, for example, hold the process lock
while > > operating on a knlist attached to that process? It???s not obvious
to me > > that this is required or even desirable. This might suggest that a
> > knlist should have its own lock rather than using a lock from the event >
> producing entity, which might make addressing this problem more > >
straightforward. > > Consider the purpose of knlist. It serves as a container
for all knotes > registered on the given subsystem object, like all knotes of
the socket, > process etc which must be fired on event. See the knote() code.
The > consequence is that the subsystem which fires knote() typically already >
holds a lock protecting its own state. As result, it is natural to > protect
the list of the knotes to activate on subsystem event, by the > subsystem lock.
> > diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c > index
0614903..3f45dca 100644 > --- a/sys/kern/kern_event.c There is not much gdb
info here; I'll try to rebuild kgdb.
https://people.freebsd.org/~pho/stress/log/kostik900.txt The number of CPUs
seems important to this test. Four works for me. - Peter
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