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

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Hiya,

On 15/04/2021 17:45, Will Deacon wrote:
On Thu, Apr 15, 2021 at 04:26:46PM +0000, Ali Saidi wrote:

On Thu, 15 Apr 2021 16:02:29 +0100, Will Deacon wrote:
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?

You're right. What we're seeing is an A-B-A problem that can allow
atomic_cond_read_acquire() to succeed and before the cmpxchg succeeds a reader
performs an A-B-A on the lock which allows the core to observe a read that
follows the cmpxchg ahead of the cmpxchg succeeding.

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);
                                      | chain_epi_lockless()
                                      |    epi->next = xchg(&ep->ovflist, epi);
                                      | read_unlock_irqrestore(&ep->lock, flags);
                                      |
          atomic_cmpxchg_relaxed()    |
   READ_ONCE(ep->ovflist);


Please stick this in the commit message, preferably annotated a bit like
Peter's example to show the READ_ONCE() being speculated.


I can confirm that this patch fixes a problem observed in
ep_scan_ready_list(.) whereby ovflist appeared to change when the write
lock was held.

So please feel free to add:
Tested-by: Steve Capper <steve.capper@xxxxxxx>

Also, I have spent a decent chunk of time looking at the above issue and
went through qrwlock, so FWIW, please feel free to add:
Reviewed-by: Steve Capper <steve.capper@xxxxxxx>

Cheers,
--
Steve



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