[PATCH 2/5] locking/rtmutex: Initialize the ww_mutex context of a waiter

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

With PREEMPT_RT no deadlock on PI futexes is reported: two threads that
each block on a PI futex the other holds sleep for good, where
FUTEX_LOCK_PI returns EDEADLK without PREEMPT_RT, and a chain deeper
than max_lock_depth is admitted.

rt_mutex_init_waiter() leaves waiter->ww_ctx as it finds it, and only
__rt_mutex_slowlock_locked() sets it. The waiters of PI futexes and of
rtlocks keep the 0x11 poison of CONFIG_DEBUG_RT_MUTEXES, or what the
stack held unless the compiler zeroes it. Since
commit 6467822b8cc9 ("locking/rtmutex: Prevent spurious EDEADLK return
caused by ww_mutexes"), the PREEMPT_RT chain walk takes a waiter with a
context for a ww_mutex one: it turns deadlock detection off and drops
the -EDEADLK of a cycle. The ww_mutex build of the walk would also
dereference the poison, in __waiter_less(), on two such waiters of equal
priority on a lock it goes through.

Clear the context with the rest of the waiter. In 8-CPU VMs with
PREEMPT_RT and CONFIG_DEBUG_RT_MUTEXES, v7.4-rc1 lets the ABBA time out
and admits the deep chain, in 2 VMs of 2; with this patch both get
EDEADLK. The futex_lock_pi_inherit selftest added later in this series
has the two cases, deadlock and max_lock_depth, and found this.

Fixes: 6467822b8cc9 ("locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes")
Cc: stable@vger.kernel.org
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/locking/rtmutex_common.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index c38b7bdea7b3..1f56d40c3e66 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -218,6 +218,7 @@ static inline void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter)
 	RB_CLEAR_NODE(&waiter->tree.entry);
 	waiter->wake_state = TASK_NORMAL;
 	waiter->task = NULL;
+	waiter->ww_ctx = NULL;
 }
 
 static inline void rt_mutex_init_rtlock_waiter(struct rt_mutex_waiter *waiter)
-- 
2.47.3