[tip: sched/core] locking: rtmutex: Fix wake_q logic in task_blocks_on_rt_mutex

[tip-bot2 for John Stultz]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     82f9cc094975240885c93effbca7f4603f5de1bf
Gitweb:        https://git.kernel.org/tip/82f9cc094975240885c93effbca7f4603f5de1bf
Author:        John Stultz <jstultz@xxxxxxxxxx>
AuthorDate:    Thu, 14 Nov 2024 11:00:47 -08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 02 Dec 2024 12:01:29 +01:00

locking: rtmutex: Fix wake_q logic in task_blocks_on_rt_mutex

Anders had bisected a crash using PREEMPT_RT with linux-next and
isolated it down to commit 894d1b3db41c ("locking/mutex: Remove
wakeups from under mutex::wait_lock"), where it seemed the
wake_q structure was somehow getting corrupted causing a null
pointer traversal.

I was able to easily repoduce this with PREEMPT_RT and managed
to isolate down that through various call stacks we were
actually calling wake_up_q() twice on the same wake_q.

I found that in the problematic commit, I had added the
wake_up_q() call in task_blocks_on_rt_mutex() around
__ww_mutex_add_waiter(), following a similar pattern in
__mutex_lock_common().

However, its just wrong. We haven't dropped the lock->wait_lock,
so its contrary to the point of the original patch. And it
didn't match the __mutex_lock_common() logic of re-initializing
the wake_q after calling it midway in the stack.

Looking at it now, the wake_up_q() call is incorrect and should
just be removed. So drop the erronious logic I had added.

Fixes: 894d1b3db41c ("locking/mutex: Remove wakeups from under mutex::wait_lock")
Closes: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@xxxxxxxxxxxxxxxx/
Reported-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Tested-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://lore.kernel.org/r/20241114190051.552665-1-jstultz@xxxxxxxxxx
---
 kernel/locking/rtmutex.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index ac1365a..e858de2 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1248,10 +1248,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
 
 		/* Check whether the waiter should back out immediately */
 		rtm = container_of(lock, struct rt_mutex, rtmutex);
-		preempt_disable();
 		res = __ww_mutex_add_waiter(waiter, rtm, ww_ctx, wake_q);
-		wake_up_q(wake_q);
-		preempt_enable();
 		if (res) {
 			raw_spin_lock(&task->pi_lock);
 			rt_mutex_dequeue(lock, waiter);