[PATCH 07/16] staging: sync: Fix a race condition between release_obj and print_obj

John Stultz
2014-02-03T18:21:28Z

From: Alistair Strachan <alistair.strachan@imgtec.com>

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.

Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>
[jstultz: minor commit subject tweak]
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
drivers/staging/android/sync.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
index fec2d1c..3d05f662 100644
--- 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);
}

--
1.8.3.2