linux-kernel - [tip: locking/urgent] rtmutex: Drop rt_mutex::wait

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Message-ID: <172372956693.2215.10534564675111967043.tip-bot2@tip-bot2>
Date: Thu, 15 Aug 2024 13:46:06 -0000
From: "tip-bot2 for Roland Xu" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Roland Xu <mu001999@...look.com>, Thomas Gleixner <tglx@...utronix.de>,
 stable@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject:
 [tip: locking/urgent] rtmutex: Drop rt_mutex::wait_lock before scheduling

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     d33d26036a0274b472299d7dcdaa5fb34329f91b
Gitweb:        https://git.kernel.org/tip/d33d26036a0274b472299d7dcdaa5fb34329f91b
Author:        Roland Xu <mu001999@...look.com>
AuthorDate:    Thu, 15 Aug 2024 10:58:13 +08:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Thu, 15 Aug 2024 15:38:53 +02:00

rtmutex: Drop rt_mutex::wait_lock before scheduling

rt_mutex_handle_deadlock() is called with rt_mutex::wait_lock held.  In the
good case it returns with the lock held and in the deadlock case it emits a
warning and goes into an endless scheduling loop with the lock held, which
triggers the 'scheduling in atomic' warning.

Unlock rt_mutex::wait_lock in the dead lock case before issuing the warning
and dropping into the schedule for ever loop.

[ tglx: Moved unlock before the WARN(), removed the pointless comment,
  	massaged changelog, added Fixes tag ]

Fixes: 3d5c9340d194 ("rtmutex: Handle deadlock detection smarter")
Signed-off-by: Roland Xu <mu001999@...look.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/all/ME0P300MB063599BEF0743B8FA339C2CECC802@ME0P300MB0635.AUSP300.PROD.OUTLOOK.COM
---
 kernel/locking/rtmutex.c |  9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 88d08ee..fba1229 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1644,6 +1644,7 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock,
 }
 
 static void __sched rt_mutex_handle_deadlock(int res, int detect_deadlock,
+					     struct rt_mutex_base *lock,
 					     struct rt_mutex_waiter *w)
 {
 	/*
@@ -1656,10 +1657,10 @@ static void __sched rt_mutex_handle_deadlock(int res, int detect_deadlock,
 	if (build_ww_mutex() && w->ww_ctx)
 		return;
 
-	/*
-	 * Yell loudly and stop the task right here.
-	 */
+	raw_spin_unlock_irq(&lock->wait_lock);
+
 	WARN(1, "rtmutex deadlock detected\n");
+
 	while (1) {
 		set_current_state(TASK_INTERRUPTIBLE);
 		rt_mutex_schedule();
@@ -1713,7 +1714,7 @@ static int __sched __rt_mutex_slowlock(struct rt_mutex_base *lock,
 	} else {
 		__set_current_state(TASK_RUNNING);
 		remove_waiter(lock, waiter);
-		rt_mutex_handle_deadlock(ret, chwalk, waiter);
+		rt_mutex_handle_deadlock(ret, chwalk, lock, waiter);
 	}
 
 	/*