Date: Wed, 15 Jun 2016 10:39:42 -0700 From: Matthew Macy <mmacy@nextbsd.org> To: "Peter Holm" <peter@holm.cc> Cc: "Konstantin Belousov" <kostikbel@gmail.com>, "Eric Badger" <eric@badgerio.us>, "freebsd-current" <freebsd-current@freebsd.org> Subject: Re: Kqueue races causing crashes Message-ID: <1555525b518.c9c704c026886.2375886287356557279@nextbsd.org> In-Reply-To: <20160615115000.GA23198@x2.osted.lan> References: <34035bf6-8b3c-d15c-765b-94bcc919ea2e@badgerio.us> <20160615081143.GS38613@kib.kiev.ua> <20160615115000.GA23198@x2.osted.lan>
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You can use dwarf4 if you use GDB from ports=C2=A0---- On Wed, =
15 Jun 2016 04:50:00 -0700 Peter Holm<peter@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 m=
ore or less the same cause. The crashes occur > > because we acquire a knl=
ist 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 ano=
ther thread. Thus we are unable to unlock the > > previously acquired lock=
and hold it until something causes us to crash > > (such as the witness c=
ode noticing that we???re returning to userland with > > the lock still he=
ld). > ... > > I believe there???s also a small window where the KN_LIST_LO=
CK 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 produ=
ce the > > ???page fault while in kernel mode??? crash. > > > > If someon=
e 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 > > t=
hought is that a ???struct knote??? ought to have its own mutex for > > co=
ntrolling 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 loc=
k, > > 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 > > ke=
rn_exit.c; the PROC_LOCK macro has already been used to acquire the > > pr=
ocess lock, also serving as the knlist lock). > This sounds as a good and c=
orrect 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 bett=
er luck reproducing them. Still, I think > that the problem is there. > > =
IMO we should simply avoid clearing kn_knlist in knlist_remove(). The > me=
mber is only used to get the locking function pointers, otherwise > code re=
lies on KN_DETACHED flag to detect on-knlist condition. See > the patch be=
low. > > > > > Apropos of the knlist lock and its provenance: why is a lo=
ck from the > > event producing entity used to control access to the knlis=
t and knote? > > Is it generally desirable to, for example, hold the proce=
ss lock while > > operating on a knlist attached to that process? It???s n=
ot obvious to me > > that this is required or even desirable. This might s=
uggest that a > > knlist should have its own lock rather than using a lock=
from the event > > producing entity, which might make addressing this pro=
blem more > > straightforward. > > Consider the purpose of knlist. It ser=
ves as a container for all knotes > registered on the given subsystem objec=
t, like all knotes of the socket, > process etc which must be fired on even=
t. See the knote() code. The > consequence is that the subsystem which fire=
s knote() typically already > holds a lock protecting its own state. As res=
ult, it is natural to > protect the list of the knotes to activate on subsy=
stem 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. h=
ttps://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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