On Tue, Apr 16, 2019 at 02:16:11PM -0400, Waiman Long wrote: > >> @@ -324,6 +364,12 @@ static void __rwsem_mark_wake(struct rw_semaphore > >> *sem, > >> adjustment -= RWSEM_FLAG_WAITERS; > >> } > >> > >> + /* > >> + * Clear the handoff flag > >> + */ > > Right, but that is a trivial comment in the 'increment i' style, it > > clearly states what the code does, but completely fails to elucidate the > > code. > > > > Maybe: > > > > /* > > * When we've woken a reader, we no longer need to force writers > > * to give up the lock and we can clear HANDOFF. > > */ > > > > And I suppose this is required if we were the pickup of the handoff set > > above, but is there a guarantee that the HANDOFF was not set by a > > writer? > > I can change the comment. The handoff bit is always cleared in > rwsem_try_write_lock() when the lock is successfully acquire. Will add a > comment to document that.
That doesn't help much, because it drops ->wait_lock between setting it and acquiring it. So the read-acquire can interleave. I _think_ it works, but I'm having trouble explaining how exactly. I think because readers don't spin yet and thus wakeups abide by queue order. And the other way around should have (write) spinners terminate the moment they see HANDOFF set by a readers, but I'm not immediately seeing that either. I'll continue staring at that.
