[PATCH v2] locking/qrwlock: Fix ordering in queued_write_lock_slowpath

[Ali Saidi <alisaidi@amazon.com>]

While this code is executed with the wait_lock held, a reader can
acquire the lock without holding wait_lock.  The writer side loops
checking the value with the atomic_cond_read_acquire(), but only truly
acquires the lock when the compare-and-exchange is completed
successfully which isn’t ordered. This exposes the window between the
acquire and the cmpxchg to an A-B-A problem which allows reads following
the lock acquisition to observe values speculatively before the write
lock is truly acquired.

We've seen a problem in epoll where the reader does a xchg while
holding the read lock, but the writer can see a value change out from under it.

Writer                               | Reader 2
--------------------------------------------------------------------------------
ep_scan_ready_list()                 |
|- write_lock_irq()                  |
    |- queued_write_lock_slowpath()  |
      |- atomic_cond_read_acquire()  |
                                     | read_lock_irqsave(&ep->lock, flags);
   --> (observes value before unlock)|  chain_epi_lockless()
   |                                 |    epi->next = xchg(&ep->ovflist, epi);
   |                                 | read_unlock_irqrestore(&ep->lock, flags);
   |                                 |
   |     atomic_cmpxchg_relaxed()    |
   |-- READ_ONCE(ep->ovflist);       |

A core can order the read of the ovflist ahead of the
atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire semantics
addresses this issue at which point the atomic_cond_read can be switched
to use relaxed semantics.

Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwlock")
Signed-off-by: Ali Saidi <alisaidi@amazon.com>
Cc: stable@vger.kernel.org
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>

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

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 4786dd271b45..10770f6ac4d9 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -73,8 +73,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
 
 	/* When no more readers or writers, set the locked flag */
 	do {
-		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
-	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
+		atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
+	} while (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
 					_QW_LOCKED) != _QW_WAITING);
 unlock:
 	arch_spin_unlock(&lock->wait_lock);
-- 
2.24.4.AMZN