[Xen-devel] [PATCH] xen: avoid deadlock in xenbus

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxx>
---
 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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