linux-kernel - [tip: locking/core] locking/ww_mutex: Initialize waiter.ww

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Date:   Fri, 20 Aug 2021 10:20:11 -0000
From:   "tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/ww_mutex: Initialize waiter.ww_ctx properly

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

Commit-ID:     b857174e68e26f9c4f0796971e11eb63ad5a3eb6
Gitweb:        https://git.kernel.org/tip/b857174e68e26f9c4f0796971e11eb63ad5a3eb6
Author:        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate:    Thu, 19 Aug 2021 21:30:30 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Fri, 20 Aug 2021 12:15:54 +02:00

locking/ww_mutex: Initialize waiter.ww_ctx properly

The consolidation of the debug code for mutex waiter intialization sets
waiter::ww_ctx to a poison value unconditionally. For regular mutexes this
is intended to catch the case where waiter_ww_ctx is dereferenced
accidentally.

For ww_mutex the poison value has to be overwritten either with a context
pointer or NULL for ww_mutexes without context.

The rework broke this as it made the store conditional on the context
pointer instead of the argument which signals whether ww_mutex code should
be compiled in or optiized out. As a result waiter::ww_ctx ends up with the
poison pointer for contextless ww_mutexes which causes a later dereference of
the poison pointer because it is != NULL.

Use the build argument instead so for ww_mutex the poison value is always
overwritten.

Fixes: c0afb0ffc06e6 ("locking/ww_mutex: Gather mutex_waiter initialization")
Reported-by: Guenter Roeck <linux@...ck-us.net>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lore.kernel.org/r/20210819193030.zpwrpvvrmy7xxxiy@linutronix.de

---
 kernel/locking/mutex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 3a65bf4..d456579 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -618,7 +618,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas

 	debug_mutex_lock_common(lock, &waiter);
 	waiter.task = current;
-	if (ww_ctx)
+	if (use_ww_ctx)
 		waiter.ww_ctx = ww_ctx;

 	lock_contended(&lock->dep_map, ip);