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From:   Waiman Long <longman@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH-tip 1/5] locking/ww_mutex: Revert "Treat ww_mutex_lock() like a trylock"
Date:   Thu, 18 Mar 2021 13:28:10 -0400
Message-Id: <20210318172814.4400-2-longman@redhat.com>
In-Reply-To: <20210318172814.4400-1-longman@redhat.com>
References: <20210318172814.4400-1-longman@redhat.com>
Precedence: bulk

It turns out that treating ww_mutex_lock() as a trylock will fail to catch
real deadlock hazard like:

	mutex_lock(&A);			ww_mutex_lock(&B, ctx);
	ww_mutex_lock(&B, ctx);		mutex_lock(&A);

The current lockdep code should be able to handle mixed lock ordering
of ww_mutexes as long as

 1) there is a top level nested lock that is acquired before hand, and
 2) the nested lock and the ww_mutex are of the same lock class.

Any ww_mutex use cases that do not provide the above guarantee will
have to be modified to avoid lockdep problem.

Revert the previous commit b058f2e4d0a7 ("locking/ww_mutex: Treat
ww_mutex_lock() like a trylock").

Fixes: commit b058f2e4d0a7 ("locking/ww_mutex: Treat ww_mutex_lock() like a trylock")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/mutex.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index bb89393cd3a2..622ebdfcd083 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -946,10 +946,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	}
 
 	preempt_disable();
-	/*
-	 * Treat as trylock for ww_mutex.
-	 */
-	mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, nest_lock, ip);
+	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
 
 	if (__mutex_trylock(lock) ||
 	    mutex_optimistic_spin(lock, ww_ctx, NULL)) {
-- 
2.18.1