[tip: locking/urgent] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN

[tip-bot2 for Waiman Long]

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

Commit-ID:     39e7234f00bc93613c086ae42d852d5f4147120a
Gitweb:        https://git.kernel.org/tip/39e7234f00bc93613c086ae42d852d5f4147120a
Author:        Waiman Long <longman@xxxxxxxxxx>
AuthorDate:    Wed, 15 Jan 2020 10:43:36 -05:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 17 Jan 2020 10:19:27 +01:00

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@xxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Christoph Hellwig <hch@xxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20200115154336.8679-1-longman@xxxxxxxxxx
---
 kernel/locking/rwsem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 44e6876..0d9b6be 100644
--- 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. */