From 82f9cc094975240885c93effbca7f4603f5de1bf Mon Sep 17 00:00:00 2001 From: John Stultz Date: Thu, 14 Nov 2024 11:00:47 -0800 Subject: [PATCH] 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@app.fastmail.com/ Reported-by: Anders Roxell Reported-by: Arnd Bergmann Signed-off-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Juri Lelli Tested-by: Anders Roxell Tested-by: K Prateek Nayak Link: https://lore.kernel.org/r/20241114190051.552665-1-jstultz@google.com --- kernel/locking/rtmutex.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index ac1365afcc4a..e858de203eb6 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);