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Subject[PATCH] xen: avoid deadlock in xenbus
Date
When starting the xenwatch thread a theoretical deadlock situation is
possible:

xs_init() contains:

task = kthread_run(xenwatch_thread, NULL, "xenwatch");
if (IS_ERR(task))
return PTR_ERR(task);
xenwatch_pid = task->pid;

And xenwatch_thread() does:

mutex_lock(&xenwatch_mutex);
...
event->handle->callback();
...
mutex_unlock(&xenwatch_mutex);

The callback could call unregister_xenbus_watch() which does:

...
if (current->pid != xenwatch_pid)
mutex_lock(&xenwatch_mutex);
...

In case a watch is firing before xenwatch_pid could be set and the
callback of that watch unregisters a watch, then a self-deadlock would
occur.

Avoid this by setting xenwatch_pid in xenwatch_thread().

Signed-off-by: Juergen Gross <jgross@suse.com>
---
drivers/xen/xenbus/xenbus_xs.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index e46080214955..3e59590c7254 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -857,6 +857,8 @@ static int xenwatch_thread(void *unused)
struct list_head *ent;
struct xs_watch_event *event;

+ xenwatch_pid = current->pid;
+
for (;;) {
wait_event_interruptible(watch_events_waitq,
!list_empty(&watch_events));
@@ -925,7 +927,6 @@ int xs_init(void)
task = kthread_run(xenwatch_thread, NULL, "xenwatch");
if (IS_ERR(task))
return PTR_ERR(task);
- xenwatch_pid = task->pid;

/* shutdown watches for kexec boot */
xs_reset_watches();
--
2.12.3
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