Revision 309657 to stack_machdep.c renders unbootable system
John Baldwin
jhb at freebsd.org
Fri Dec 16 23:19:13 UTC 2016
On Wednesday, December 14, 2016 04:50:21 PM Mark Johnston wrote:
> On Wed, Dec 14, 2016 at 03:48:04PM -0800, Steven G. Kargl wrote:
> > On Wed, Dec 14, 2016 at 02:10:48PM -0800, Mark Johnston wrote:
> > > On Wed, Dec 14, 2016 at 12:14:16PM -0800, Mark Johnston wrote:
> > > > On Wed, Dec 14, 2016 at 11:49:26AM -0800, Steven G. Kargl wrote:
> > > > > Well, after 3 days of bisection, I finally found the commit
> > > > > that renders my system unbootable. The system does not panic.
> > > > > It simply gets stuck in some state. Nonfunctional keyboard,
> > > > > so can't break into debugger. No serial console available.
> > > > > The verbose dmesg.boot for a working kernel from revision
> > > > > 309656 is at
> > > > >
> > > > > http://troutmask.apl.washington.edu/~kargl/freebsd/dmesg.309656.txt
> > > > >
> > > > > The kernel config file is at
> > > > >
> > > > > http://troutmask.apl.washington.edu/~kargl/freebsd/SPEW.txt
> > > > >
> > > > > In looking at /usr/src/UPDATING, there is no warning that one
> > > > > can create a boat anchor by upgrading to 309657. If compiling
> > > > > a kernel with 'options DDB' is no longer supported, this should
> > > > > be stated in UPDATING. Or, UPDATING should state that 'options
> > > > > DDB' requires 'options STACK'. Or, 'options DDB' should simply
> > > > > to the right thing and pull in whatever 'option STACK' does.
> > > >
> > > > It is supported though - the point of that change was to fix a problem
> > > > that occurred when DDB is configured but STACK isn't. While testing I
> > > > tried every combination of the two options, and I just tried and
> > > > successfully booted a kernel with DDB and !STACK.
> > > >
> > > > Does the kernel boot successfully if STACK is added to your
> > > > configuration?
> > >
> > > I tried your config (plus virtio drivers) and was able to reproduce the
> > > hang in bhyve. Adding STACK "fixed" the hang, as did reverting part of
> > > my change to re-add dead code into the kernel. My VM was always hanging
> > > after printing
> > >
> > > 000.000050 [ 426] vtnet_netmap_attach virtio attached txq=1, txd=1024 rxq=1, rxd=1024
> > >
> > > Sure enough, removing "device netmap" from your config also fixes the
> > > hang. When the hang occurs, I can see with "bhyvectl --get-rip" that
> > > we're stuck in DELAY(), but I can't get a stack at that point. I think
> > > my change is an innocent bystander - it just happened to expose a latent
> > > issue elsewhere.
> > >
> > > I don't have much more time to look at this right now, but I'll look
> > > into it more tonight.
> >
> > Yes, adding STACK got me to a booting kernel. I can't remember
> > why I added netmap to my config file. Re-adding dead code seems to
> > point to some memory corruption issue or a rogue pointer. :(
>
> It's not quite that bad, as it turns out. The key is that
> adding/removing the dead code changes the ordering of the items in the
> sysinit linker set. I discovered that if the ctl(4) module is
> initialized before the vtnet driver attaches, the hang occurs, and
> reverting my commit results in a sysinit order where vtnet comes
> _before_ ctl(4). So my change triggers the problem just because it
> happens to perturb something in the compile-time linker.
>
> >
> > BTW, I think it would be prudent to add something like
> >
> > 20161206:
> > At revision 309657, 'options STACK' was introduced into
> > sys/x86/x86/mstack_machdep.c. Old kernel configuration files
> > that included 'options DDB' are now required to include also
> > 'options STACK'.
> >
> > to UPDATING or some such wording. I was jumping from circ Oct 10th world
> > to top of tree, and got caught by ~3000 commits.
>
> The issue actually seems to be in 4BSD, and more specifically in r308564
> and r308565. Switching to ULE or reverting either of those two commits
> fixes the hang. Here's what happens:
>
> 1. ctl_init() runs and creates ctl_thresh_thread. This thread's main
> loop cause pause(9) when it has no work to do. During boot, pause(9)
> just calls DELAY() and does not yield the CPU.
> 2. thread0 attaches the vtnet driver. As part of this, it creates and
> starts some high-priority taskqueue threads in
> _taskqueue_thread_start(). They're added to the scheduler with:
>
> thread_lock();
> sched_pri(...);
> sched_add(...);
> thread_unlock();
>
> 4BSD's sched_add() will call maybe_preempt() in this case, which as
> of r308564 will unconditionally set td_owepreempt in the current
> thread.
> 3. thread_unlock() will release the critical section held by the current
> thread and because td_owepreempt is set, we'll yield the CPU. The
> taskqueue threads have nothing to do, but ctl_thresh_thread runs
> and ends up busy-waiting in pause() forever.
>
> r308565 removes a check in maybe_preempt() that would have stopped
> td_owepreempt from being set. Before r308564, maybe_preempt() would have
> switched directly to the new thread and apparently always switched back
> immediately.
>
> I'm not sure what the correct fix is - jhb might have an idea. I wonder
> if pause() should try to yield periodically when called during boot.
So the hack in pause() is probably not as necessary now. In particular, I
think we only need it for thread0, not for other threads. The patch below
worked for me with SPEW's config:
Index: kern_synch.c
===================================================================
--- kern_synch.c (revision 310128)
+++ kern_synch.c (working copy)
@@ -321,7 +321,8 @@ pause_sbt(const char *wmesg, sbintime_t sbt, sbint
if (sbt == 0)
sbt = tick_sbt;
- if (cold || kdb_active || SCHEDULER_STOPPED()) {
+ if ((cold && curthread == &thread0) || kdb_active ||
+ SCHEDULER_STOPPED()) {
/*
* We delay one second at a time to avoid overflowing the
* system specific DELAY() function(s):
--
John Baldwin
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