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
