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From: Yuriy Tsibizov <Yuriy.Tsibizov@gfk.ru>
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Cc: current@freebsd.org, kmacy@freebsd.org
Subject: Re: Fatal trap at booting after last kmacy@ changes
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> Last kernel got immediately fatal trap at boot, right after timer probe.
> This is what I write by hand from the console:
>
> Fatal trap 12: page fault while in kernel mode
> fault code: supervisor read, page not present
> stopped at sleepq_add+0xf6: cmpl $0,0(%rax,%eax,8)
>
> stack trace:
> sleepq_add
> cv_timedwait
> _sema_timedwait
> ata_queue_request
> ...

It seems that with INVARIANTS &sq is referenced while sq is NULL...

This should fix it:

Index: subr_sleepqueue.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/subr_sleepqueue.c,v
retrieving revision 1.32
diff -u -r1.32 subr_sleepqueue.c
--- subr_sleepqueue.c	16 Dec 2006 07:50:39 -0000	1.32
+++ subr_sleepqueue.c	16 Dec 2006 17:34:27 -0000
@@ -295,6 +295,7 @@
  	 * into the sleep queue already in use by this wait channel.
  	 */
  	if (sq == NULL) {
+		sq = td->td_sleepqueue;
  #ifdef INVARIANTS
  		int i;
  		for (i = 0; i < NR_SLEEPQS; i++)
@@ -313,7 +314,6 @@
  				sleepq_max_depth = sc->sc_max_depth;
  		}
  #endif
-		sq = td->td_sleepqueue;
  		LIST_INSERT_HEAD(&sc->sc_queues, sq, sq_hash);
  		sq->sq_wchan = wchan;
  #ifdef INVARIANTS

Yuriy.