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

[Peter Zijlstra]

I've edited the thing to look like so. I'll go queue it for
locking/urgent.

---
Subject: locking/qrwlock: Fix ordering in queued_write_lock_slowpath()
From: Ali Saidi <alisaidi@xxxxxxxxxx>
Date: Thu, 15 Apr 2021 17:27:11 +0000

From: Ali Saidi <alisaidi@xxxxxxxxxx>

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
  --------------------------------------------------------------------------------
  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>
[peterz: use try_cmpxchg()]
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Steve Capper <steve.capper@xxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Tested-by: Steve Capper <steve.capper@xxxxxxx>
---
 kernel/locking/qrwlock.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -60,6 +60,8 @@ EXPORT_SYMBOL(queued_read_lock_slowpath)
  */
 void queued_write_lock_slowpath(struct qrwlock *lock)
 {
+	int cnts;
+
 	/* Put the writer into the wait queue */
 	arch_spin_lock(&lock->wait_lock);
 
@@ -73,9 +75,8 @@ void queued_write_lock_slowpath(struct q
 
 	/* 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,
-					_QW_LOCKED) != _QW_WAITING);
+		cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
+	} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
 unlock:
 	arch_spin_unlock(&lock->wait_lock);
 }