linux-kernel - [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER

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Message-Id: <20200114190303.5778-1-longman@redhat.com>
Date:   Tue, 14 Jan 2020 14:03:03 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        stable@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN

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@....de>
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/rwsem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 44e68761f432..1dd3d53f43c3 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1227,7 +1227,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 		 * without sleeping.
 		 */
 		if ((wstate == WRITER_HANDOFF) &&
-		    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
+		    rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
 			goto trylock_again;
 
 		/* Block until there are no active lockers. */
-- 
2.18.1