cvs commit: src/sys/kern kern_mutex.c

[Kostik Belousov]

On Tue, Jun 05, 2007 at 05:43:03PM +0200, Attilio Rao wrote:
> 2007/6/5, Attilio Rao <attilio at freebsd.org>:
> >2007/6/5, Bruce Evans <brde at 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?
> 
> 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:
> 
> td = curthread;
> if (td->td_critnest > 0 || td1->td_priority >= td->td_priority)
>         return;
> 
> thread_lock(td1);
> if (!TD_IS_RUNNING(td1)) {
> ...
> 
> mi_switch(SW_INVOL, NULL);
> ...
> }
> thread_unlock(td1);
> 
> Which is wrong beacause td1 is not curthread and really curthread
> should be locked too when context switching.
> 
> 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=%p", m,
 			    (void *)m->mtx_lock);
 
+		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=%p",
 			    m, (void *)m->mtx_lock);
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