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
