[src/trunk]: src/sys/arch/xen/xenbus xen: prevent adding duplicate xenwatches

[royger <royger%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/432d7f8e877b
branches:  trunk
changeset: 782952:432d7f8e877b
user:      royger <royger%NetBSD.org@localhost>
date:      Wed Nov 28 16:26:59 2012 +0000

description:
xen: prevent adding duplicate xenwatches

When a xenstore watch triggers, the event is processed on process_msg
and if a valid handle it's found the handler is queued for execution
on the pending xen watches queue (watch_events).

This may present a problem if we trigger a xenwatch several times and
then disconnect the device. If several xenwatch events are added to
the watch_events queue and the device is disconnected afterwards, the
first processed xenwatch event will disconnect the device, remove the
watch and free all resources. This triggers a panic if there are
pending xenwatch events for that device already queued in the local
queue of the function xenwatch_thread, since when the next watch that
has the same handler tries to execute we get a panic due to the fact
that the device is already disconnected and all resources had been
freed:

xenbus_watch: 0xffffa0000b7cd1d0
xbw_callback: 0xffffffff80755dd4
otherend_changed: backend/vif/1/0
backend/vif/1/0/state 6
backend/vif/1/0 -> Closed
backend/vif/1/0 -> backend_device, b_detach: 0xffffffff8075a2bf
xenbus_watch: 0xffffa0000b7cd1d0
xbw_callback: 0xfc5ec02183e547a8
fatal protection fault in supervisor mode
trap type 4 code 0 rip ffffffff80756596 cs e030 rflags 10246 cr2
7f7ff7b4c020 ilevel 0 rsp ffffa000e6d82c50
curlwp 0xffffa0000a72d580 pid 0 lid 36 lowest kstack
0xffffa000e6d7f000
kernel: protection fault trap, code=0
Stopped in pid 0.36 (system) at netbsd:xenwatch_thread+0xc7:    call
*10(%rax
)
xenwatch_thread() at netbsd:xenwatch_thread+0xc7
ds          f
es          5987
fs          2c40
gs          1460
rdi         ffffa0000b7cd1d0
rsi         ffffa0000a5477f0
rbp         ffffa000e6d82c70
rbx         ffffa0000b7c14c0
rdx         2
rcx         f
rax         ffffa0000b7cd1d0
r8          78
r9          ffffffef
r10         deadbeef
r11         1
r12         ffffa000e6d82c50
r13         ffffa0000a72d580
r14         ffffa0000a72d580
r15         0
rip         ffffffff80756596    xenwatch_thread+0xc7
cs          e030
rflags      10246
rsp         ffffa000e6d82c50
ss          e02b
netbsd:xenwatch_thread+0xc7:    call    *10(%rax)

diffstat:

 sys/arch/xen/xenbus/xenbus_xs.c |  24 +++++++++++++++++-------
 1 files changed, 17 insertions(+), 7 deletions(-)

diffs (64 lines):

diff -r a61b42acc12f -r 432d7f8e877b sys/arch/xen/xenbus/xenbus_xs.c
--- a/sys/arch/xen/xenbus/xenbus_xs.c   Wed Nov 28 13:34:24 2012 +0000
+++ b/sys/arch/xen/xenbus/xenbus_xs.c   Wed Nov 28 16:26:59 2012 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: xenbus_xs.c,v 1.22 2011/07/27 23:11:23 matt Exp $ */
+/* $NetBSD: xenbus_xs.c,v 1.23 2012/11/28 16:26:59 royger Exp $ */
 /******************************************************************************
  * xenbus_xs.c
  *
@@ -30,7 +30,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xenbus_xs.c,v 1.22 2011/07/27 23:11:23 matt Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xenbus_xs.c,v 1.23 2012/11/28 16:26:59 royger Exp $");
 
 #if 0
 #define DPRINTK(fmt, args...) \
@@ -751,7 +751,7 @@
 static int
 process_msg(void)
 {
-       struct xs_stored_msg *msg;
+       struct xs_stored_msg *msg, *s_msg;
        char *body;
        int err;
 
@@ -782,7 +782,7 @@
        body[msg->hdr.len] = '\0';
 
        if (msg->hdr.type == XS_WATCH_EVENT) {
-               bool found;
+               bool found, repeated;
 
                DPRINTK("process_msg: XS_WATCH_EVENT");
                msg->u.watch.vec = split(body, msg->hdr.len,
@@ -796,14 +796,24 @@
                msg->u.watch.handle = find_watch(
                    msg->u.watch.vec[XS_WATCH_TOKEN]);
                found = (msg->u.watch.handle != NULL);
+               repeated = false;
                if (found) {
                        mutex_enter(&watch_events_lock);
-                       SIMPLEQ_INSERT_TAIL(&watch_events, msg, msg_next);
-                       cv_broadcast(&watch_cv);
+                       /* Don't add duplicate events to the queue of pending watches */
+                       SIMPLEQ_FOREACH(s_msg, &watch_events, msg_next) {
+                               if (s_msg->u.watch.handle == msg->u.watch.handle) {
+                                       repeated = true;
+                                       break;
+                               }
+                       }
+                       if (!repeated) {
+                               SIMPLEQ_INSERT_TAIL(&watch_events, msg, msg_next);
+                               cv_broadcast(&watch_cv);
+                       }
                        mutex_exit(&watch_events_lock);
                }
                mutex_exit(&watches_lock);
-               if (!found) {
+               if (!found || repeated) {
                        free(msg->u.watch.vec, M_DEVBUF);
                        free(msg, M_DEVBUF);
                }