Messages in this thread Patch in this message | | | Date | Thu, 15 Apr 2021 16:02:29 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] locking/qrwlock: Fix ordering in queued_write_lock_slowpath |
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On Thu, Apr 15, 2021 at 02:25:52PM +0000, 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. The other atomic operations from this > point are release-ordered and thus reads after the lock acquisition can > be completed before the lock is truly acquired which violates the > guarantees the lock should be making.
I think it would be worth spelling this out with an example. The issue appears to be a concurrent reader in interrupt context taking and releasing the lock in the window where the writer has returned from the atomic_cond_read_acquire() but has not yet performed the cmpxchg(). Loads can be speculated during this time, but the A-B-A of the lock word from _QW_WAITING to (_QW_WAITING | _QR_BIAS) and back to _QW_WAITING allows the atomic_cmpxchg_relaxed() to succeed. Is that right?
With that in mind, it would probably be a good idea to eyeball the qspinlock slowpath as well, as that uses both atomic_cond_read_acquire() and atomic_try_cmpxchg_relaxed().
> Fixes: b519b56e378ee ("locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwloc")
Typo in the quoted subject ('qrwloc').
> Signed-off-by: Ali Saidi <alisaidi@amazon.com> > Cc: stable@vger.kernel.org > --- > 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);
Patch looks good, so with an updated message:
Acked-by: Will Deacon <will@kernel.org>
Will
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