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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Acked-by: Will Deacon <will@xxxxxxxxxx>
Tested-by: Steve Capper <steve.capper@xxxxxxx>
Reviewed-by: Steve Capper <steve.capper@xxxxxxx>
---
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
