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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Waiman Long <longman@redhat.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: [PATCH 5.4 064/222] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN
Date:   Wed, 22 Jan 2020 10:27:30 +0100
Message-Id: <20200122092838.302318765@linuxfoundation.org>
In-Reply-To: <20200122092833.339495161@linuxfoundation.org>
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From: Waiman Long <longman@redhat.com>

commit 39e7234f00bc93613c086ae42d852d5f4147120a upstream.

The commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer
optimistically spin on owner") will allow a recently woken up waiting
writer to spin on the owner. Unfortunately, if the owner happens to be
RWSEM_OWNER_UNKNOWN, the code will incorrectly spin on it leading to a
kernel crash. This is fixed by passing the proper non-spinnable bits
to rwsem_spin_on_owner() so that RWSEM_OWNER_UNKNOWN will be treated
as a non-spinnable target.

Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20200115154336.8679-1-longman@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/locking/rwsem.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1226,8 +1226,8 @@ wait:
 		 * In this case, we attempt to acquire the lock again
 		 * without sleeping.
 		 */
-		if ((wstate == WRITER_HANDOFF) &&
-		    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
+		if (wstate == WRITER_HANDOFF &&
+		    rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
 			goto trylock_again;
 
 		/* Block until there are no active lockers. */