Re: panic: non-current pmap 0xffffa00020eab8f0 on Rpi3

On Fri, Oct 16, 2020 at 11:53:56AM +0200, Michal Meloun wrote:
> 
> 
> On 06.10.2020 15:37, Mark Johnston wrote:
> > On Mon, Oct 05, 2020 at 07:10:29PM -0700, bob prohaska wrote:
> >> Still seeing non-current pmap panics on the Pi3, this time a B+ running
> >> 13.0-CURRENT (GENERIC-MMCCAM) #0 71e02448ffb-c271826(master)
> >> during a -j4 buildworld.  The backtrace reports
> >>
> >> panic: non-current pmap 0xffffa00020eab8f0
> > 
> > Could you show the output of "show procvm" from the debugger?
> 
> I see same panic too, in my case its very rare - typical scenario is
> rebuild of kf5 ports (~250, 2 days of full load).  Any idea how to debug
> this?
> Michal

I suspect that there is some race involving the pmap switching in
vmspace_exit(), but I can't see it.  In the example below, presumably
process 22604 on CPU 0 is also exiting?  Could you show the backtrace?

It would also be useful to see the value of PCPU_GET(curpmap) at the
time of the panic.  I'm not sure if there's a way to get that from DDB,
but I suspect it should be equal to &vmspace0->vm_pmap.

I think vmspace_exit() should issue a release fence with the cmpset and
an acquire fence when handling the refcnt == 1 case, but I don't see why
that would make a difference here.  So, if you can test a debug patch,
this one will yield a bit more debug info.  If you can provide access to
a vmcore and kernel debug symbols, that'd be even better.

diff --git a/sys/arm64/arm64/pmap.c b/sys/arm64/arm64/pmap.c
index 284f00b3cc0d..3c53ae3b4c1e 100644
--- a/sys/arm64/arm64/pmap.c
+++ b/sys/arm64/arm64/pmap.c
@@ -4838,7 +4838,8 @@ pmap_remove_pages(pmap_t pmap)
 	int allfree, field, freed, idx, lvl;
 	vm_paddr_t pa;
 
-	KASSERT(pmap == PCPU_GET(curpmap), ("non-current pmap %p", pmap));
+	KASSERT(pmap == PCPU_GET(curpmap),
+	    ("non-current pmap %p %p", pmap, PCPU_GET(curpmap)));
 
 	lock = NULL;
 
diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c
index c20005ae64cf..0ad415e3b88c 100644
--- a/sys/vm/vm_map.c
+++ b/sys/vm/vm_map.c
@@ -358,7 +358,10 @@ vmspace_exit(struct thread *td)
 	p = td->td_proc;
 	vm = p->p_vmspace;
 	atomic_add_int(&vmspace0.vm_refcnt, 1);
-	refcnt = vm->vm_refcnt;
+	refcnt = atomic_load_int(&vm->vm_refcnt);
+
+	KASSERT(vmspace_pmap(vm) == PCPU_GET(curpmap),
+	    ("non-current pmap %p %p", pmap, PCPU_GET(curpmap)));
 	do {
 		if (refcnt > 1 && p->p_vmspace != &vmspace0) {
 			/* Switch now since other proc might free vmspace */
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