xen: avoid deadlock in xenbus

PATCH In review Juergen Gross 2 participants 5 messages Jul 28, 2017 Xen hypervisor interface

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Juergen Gross

author maintainer

2017-07-28 14:54 UTC #0 lore raw

PATCH 1/1 xen: avoid deadlock in xenbus

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 <[email protected]>

---
 drivers/xen/xenbus/xenbus_xs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

drivers/xen/xenbus/xenbus_xs.c +2 -1

index e46080214955..3e59590c7254 100644
@@ -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();

Signature · 1 line

2.12.3

Boris Ostrovsky

Reviewed-by on v1 1/1

2017-07-28 15:13 UTC #1 lore raw

Quoted 25 lines · Juergen Gross

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);
    ...

Quoted 7 lines

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 <[email protected]>

Reviewed-by Boris Ostrovsky <[email protected]>

+stable?

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Boris Ostrovsky

Reviewed-by on v1 1/1

2017-07-28 15:13 UTC #2 lore raw

In reply to Juergen Gross

Quoted 25 lines · Juergen Gross

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);
    ...

Quoted 7 lines

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 <[email protected]>

Reviewed-by Boris Ostrovsky <[email protected]>

+stable?

Juergen Gross

author maintainer

2017-07-28 15:16 UTC #3 lore raw

Quoted 30 lines · Boris Ostrovsky

Quoted 29 lines · Juergen Gross
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.

Quoted 7 lines

Quoted 3 lines
Avoid this by setting xenwatch_pid in xenwatch_thread().
Signed-off-by Juergen Gross <[email protected]>
Reviewed-by Boris Ostrovsky <[email protected]>
+stable?

As this problem is purely theoretical, I don't think the patch is appropriate for stable (at least the stable rules tell me so).

Juergen

Juergen Gross

author maintainer

2017-07-28 15:16 UTC #4 lore raw

In reply to Boris Ostrovsky

Quoted 30 lines · Boris Ostrovsky

Quoted 29 lines · Juergen Gross
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.

Quoted 7 lines

Quoted 3 lines
Avoid this by setting xenwatch_pid in xenwatch_thread().
Signed-off-by Juergen Gross <[email protected]>
Reviewed-by Boris Ostrovsky <[email protected]>
+stable?

As this problem is purely theoretical, I don't think the patch is appropriate for stable (at least the stable rules tell me so).

Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
https://lists.xen.org/xen-devel