[PATCH RT 1/7] rtmutex: Handle non enqueued waiters gracefully

From: Steven Rostedt
Date: Fri Feb 26 2016 - 17:31:50 EST

3.2.77-rt111-rc1 stable review patch.
If anyone has any objections, please let me know.


From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Yimin debugged that in case of a PI wakeup in progress when
rt_mutex_start_proxy_lock() calls task_blocks_on_rt_mutex() the latter
returns -EAGAIN and in consequence the remove_waiter() call runs into
a BUG_ON() because there is nothing to remove.

Guard it with rt_mutex_has_waiters(). This is a quick fix which is
easy to backport. The proper fix is to have a central check in
remove_waiter() so we can call it unconditionally.

Reported-and-debugged-by: Yimin Deng <yimin11.deng@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable-rt@xxxxxxxxxxxxxxx
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
kernel/rtmutex.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 52cab2720a06..f074834c1ad6 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -1935,7 +1935,7 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
ret = 0;

- if (unlikely(ret))
+ if (ret && rt_mutex_has_waiters(lock))
remove_waiter(lock, waiter);