Patch for boot-time USB hangs in 4.9-PRERELEASE

[Ian Dowse]

Could people who are experiencing boot-time hangs in 4.9-PRERELEASE
try the following patch to see if it helps? I've had one positive
report so far, but it would be helpful to get more feedback to
determine if this is the right fix to be committed.

The problem was that interrupts were getting unmasked too early in
the boot process, causing an interrupt storm that usually occurred
while USB devices were being probed. The bug is in fork1(), which
is used by kthread_create() to create kernel threads; the code there
assumed that all interrupts would be unmasked when called, so it
didn't bother saving and restoring the interrupt mask. This assumption
is reasonable for normal fork() calls, but not for the creation of
kernel threads early in the boot process.

The appearance of the problem was linked to some recent changes in
sys/kern/subr_taskqueue.c around September 10th. Those changes
include a call to kthread_create() that happens just before probing
all the devices at boot time, so the result was that interrupts
were being left unmasked at a time when they are supposed to be
disabled. To actually get an interrupt storm hang, you need some
IRQ number to be configured by a device driver, and then for some
(possibly other) device sharing that IRQ line to generate an interrupt
before the driver for that device is prepared to handle it. As well
as the subr_taskqueue.c case, there is also a call to kthread_create()
in the USB code, so that might be related too.

Apply the patch in /usr/src/sys/kern, then rebuild the kernel and
reboot. In case of whitespace problems preventing patch applying,
it is also available at:

	http://people.freebsd.org/~iedowse/fork.diff

Ian

Index: kern_fork.c
===================================================================
RCS file: /home/iedowse/CVS/src/sys/kern/kern_fork.c,v
retrieving revision 1.72.2.14
diff -u -r1.72.2.14 kern_fork.c
--- kern_fork.c	26 Jun 2003 04:15:10 -0000	1.72.2.14
+++ kern_fork.c	26 Sep 2003 08:26:31 -0000
@@ -183,7 +183,7 @@
 	struct proc *p2, *pptr;
 	uid_t uid;
 	struct proc *newproc;
-	int ok;
+	int ok, s;
 	static int curfail = 0, pidchecked = 0;
 	static struct timeval lastfail;
 	struct forklist *ep;
@@ -544,10 +544,10 @@
 	 */
 	microtime(&(p2->p_stats->p_start));
 	p2->p_acflag = AFORK;
-	(void) splhigh();
+	s = splhigh();
 	p2->p_stat = SRUN;
 	setrunqueue(p2);
-	(void) spl0();
+	splx(s);
 
 	/*
 	 * Now can be swapped.