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

[Steve Capper <steve.capper@arm.com>]

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@arm.com>

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@arm.com>

Cheers,
-- 
Steve