Re: [PATCH] workqueue: Fix memory ordering race in queue_work*()

[Hector Martin]

On 16/08/2022 23.55, Boqun Feng wrote:
> On Tue, Aug 16, 2022 at 02:41:57PM +0100, Will Deacon wrote:
>> It's worth noting that with the spinlock-based implementation (i.e.
>> prior to e986a0d6cb36) then we would have the same problem on
>> architectures that implement spinlocks with acquire/release semantics;
>> accesses from outside of the critical section can drift in and reorder
>> with each other there, so the conversion looked legitimate to me in
>> isolation and I vaguely remember going through callers looking for
>> potential issues. Alas, I obviously missed this case.
>>
> 
> I just to want to mention that although spinlock-based atomic bitops
> don't provide the full barrier in test_and_set_bit(), but they don't
> have the problem spotted by Hector, because test_and_set_bit() and
> clear_bit() sync with each other via locks:
> 
> 	test_and_set_bit():
> 	  lock(..);
> 	  old = *p; // mask is already set by other test_and_set_bit()
> 	  *p = old | mask;
> 	  unlock(...);
> 				clear_bit():
> 				  lock(..);
> 				  *p ~= mask;
> 				  unlock(..);
> 
> so "having a full barrier before test_and_set_bit()" may not be the
> exact thing we need here, as long as a test_and_set_bit() can sync with
> a clear_bit() uncondiontally, then the world is safe. For example, we
> can make test_and_set_bit() RELEASE, and clear_bit() ACQUIRE on ARM64:
> 
> 	test_and_set_bit():
> 	  atomic_long_fetch_or_release(..); // pair with clear_bit()
> 	  				    // guarantee everything is
> 					    // observed.
> 	  			clear_bit():
> 				  atomic_long_fetch_andnot_acquire(..);
> 	  
> , maybe that's somewhat cheaper than a full barrier implementation.
> 
> Thoughts? Just to find the exact ordering requirement for bitops.

It's worth pointing out that the workqueue code does *not* pair
test_and_set_bit() with clear_bit(). It does an atomic_long_set()
instead (and then there are explicit barriers around it, which are
expected to pair with the implicit barrier in test_and_set_bit()). If we
define test_and_set_bit() to only sync with clear_bit() and not
necessarily be a true barrier, that breaks the usage of the workqueue code.

- Hector