From: Boqun Feng > Sent: 02 January 2024 18:54 > > On Sat, Dec 30, 2023 at 03:49:52PM +0000, David Laight wrote: > [...] > > But it looks odd that osq_unlock()'s fast path uses _release but the very > > similar code in osq_wait_next() uses _acquire. > > > > The _release in osq_unlock() is needed since unlocks are needed to be > RELEASE so that lock+unlock can be a critical section (i.e. no memory > accesses can escape). When osq_wait_next() is used in non unlock cases, > the RELEASE is not required. As for the case where osq_wait_next() is > used in osq_unlock(), there is a xchg() preceding it, which provides a > full barrier, so things are fine. I know there have been issues with ACQUIRE/RELEASE/FULL xchg in this code, but are FULL xchg always needed on node->next? > /me wonders whether we can relax the _acquire in osq_wait_next() into > a _relaxed. I wouldn't have worried about relaxed v release. > > Indeed, apart from some (assumed) optimisations, I think osq_unlock() > > could just be: > > next = osq_wait_next(lock, this_cpu_ptr(&osq_node), 0); > > if (next) > > next->locked = 1; > > > > If so we need to provide some sort of RELEASE semantics for the > osq_unlock() in all the cases. I wonder how often the unqueue code happens, and especially for the last cpu in the list? I'd only expect need_resched() to return true after spinning for a while - in which case perhaps it is more likely that there are a lot of cpu in the queue and the cpu being removed won't be last. So osq_wait_next() exits on xchg(&node->next, NULL) != NULL which is full barrier. On a slightly different note I've also wondered if 'osq_node' actually needs to be cache line aligned? You definitely don't want it spanning 2 cache line, but I'd expect that per-cpu data is mostly accessed by its own cpu? So you really aren't going to get false sharing with some other per-cpu data since the cpu is busy in this code. So __aligned(16) would do? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
