On 4/15/21 1:27 PM, Ali Saidi wrote:
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);
Acked-by: Waiman Long longman@redhat.com