sync: Fix a race condition between release_obj and print_obj

Before this change, a timeline would only be removed from the timeline
list *after* the sync driver had its release_obj() called. However, the
driver's release_obj() may free resources needed by print_obj().

Although the timeline list is locked when print_obj() is called, it is
not locked when release_obj() is called. If one CPU was in print_obj()
when another was in release_obj(), the print_obj() may make unsafe
accesses.

It is not actually necessary to hold the timeline list lock when calling
release_obj() if the call is made after the timeline is unlinked from
the list, since there is no possibility another thread could be in --
or enter -- print_obj() for that timeline.

This change moves the release_obj() call to after the timeline is
unlinked, preventing the above race from occurring.

Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>
(cherry picked from commit 3bfc1e9e0c7d0c0e3f52ee1f3803482416afa295)

Change-Id: Ib2b2b23fde5be34016f9d86b8b9c5e561b56cd4c

diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
index c5a43934e855a84b13d83c29bb6ecb96da99dedd..d38305b409306a95046023ba77af7bb7f3f7e5bf 100644 (file)
--- a/drivers/staging/android/sync.c
+++ b/drivers/staging/android/sync.c
@@ -79,13 +79,13 @@ static void sync_timeline_free(struct kref *kref)
                container_of(kref, struct sync_timeline, kref);
        unsigned long flags;
 
-       if (obj->ops->release_obj)
-               obj->ops->release_obj(obj);
-
        spin_lock_irqsave(&sync_timeline_list_lock, flags);
        list_del(&obj->sync_timeline_list);
        spin_unlock_irqrestore(&sync_timeline_list_lock, flags);
 
+       if (obj->ops->release_obj)
+               obj->ops->release_obj(obj);
+
        kfree(obj);
 }