Date: Tue, 5 Jun 2007 19:47:40 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Attilio Rao <attilio@freebsd.org> Cc: cvs-src@freebsd.org, src-committers@freebsd.org, cvs-all@freebsd.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: cvs commit: src/sys/kern kern_mutex.c Message-ID: <20070605164740.GY2268@deviant.kiev.zoral.com.ua> In-Reply-To: <3bbf2fe10706050843x5aaafaafy284e339791bcfe42@mail.gmail.com> References: <200706051420.l55EKEih018925@repoman.freebsd.org> <20070606003412.W35544@besplex.bde.org> <20070605150005.GW2268@deviant.kiev.zoral.com.ua> <20070605150521.GX2268@deviant.kiev.zoral.com.ua> <20070606011022.R35623@besplex.bde.org> <3bbf2fe10706050829o2d756a4cu22f98cf11c01f5e4@mail.gmail.com> <3bbf2fe10706050843x5aaafaafy284e339791bcfe42@mail.gmail.com>
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On Tue, Jun 05, 2007 at 05:43:03PM +0200, Attilio Rao wrote:
> 2007/6/5, Attilio Rao <attilio@freebsd.org>:
> >2007/6/5, Bruce Evans <brde@optusnet.com.au>:
> >>
> >> I get a "spin lock held too long" panic during (an interrupt in?) acpi
> >> initialization on booting non-PREEMPTION SCHED_4BSD SMP. Haven't tried
> >> other cases.
> >
> >Do you have a backtrace or any other debugging stuffs available?
>=20
> Mmm, I think I got the bug.
> basically, in kern_mutex.c::_mtx_unlock_sleep(), in the not-preemptive
> case what happens at some point is:
>=20
> td =3D curthread;
> if (td->td_critnest > 0 || td1->td_priority >=3D td->td_priority)
> return;
>=20
> thread_lock(td1);
> if (!TD_IS_RUNNING(td1)) {
> ...
>=20
> mi_switch(SW_INVOL, NULL);
> ...
> }
> thread_unlock(td1);
>=20
> Which is wrong beacause td1 is not curthread and really curthread
> should be locked too when context switching.
>=20
> To a first look the idea is that td and td1 should be locked both, but
> I just want more time to look better at it.
With the following patch, I get the
panic: mutex sched lock recursed at .../kern_synch.c:379
with backtrace
mi_switch
_mtx_unlock_sleep
_mtx_unlock_flags
vm_pagezero
diff --git a/sys/kern/kern_mutex.c b/sys/kern/kern_mutex.c
index e0592a8..bf44229 100644
--- a/sys/kern/kern_mutex.c
+++ b/sys/kern/kern_mutex.c
@@ -670,7 +670,9 @@ _mtx_unlock_sleep(struct mtx *m, int opts, const char *=
file, int line)
"_mtx_unlock_sleep: %p switching out lock=3D%p", m,
(void *)m->mtx_lock);
=20
+ thread_lock(td);
mi_switch(SW_INVOL, NULL);
+ thread_unlock(td);
if (LOCK_LOG_TEST(&m->lock_object, opts))
CTR2(KTR_LOCK, "_mtx_unlock_sleep: %p resuming lock=3D%p",
m, (void *)m->mtx_lock);
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