On 04/29, Paul E. McKenney wrote: > > On Mon, Apr 29, 2019 at 06:06:04PM +0200, Oleg Nesterov wrote: > > > > Well, at least WRITE_ONCE()'s look certainly unneeded to me, gp_state > > is protected by rss_lock. > > > > WARN_ON_ONCE(gp_state) can read gp_state lockless, but even in this case > > I do not understand what READ_ONCE() tries to prevent... > > > > Nevermind, this won't hurt and as I already said I don't understand the > > _ONCE() magic anyway ;) > > If I understand correctly, rcu_sync_is_idle() can be inline and returns > ->gp_state.
Ah, sorry! I didn't mean rcu_sync_is_idle(). To be honeest, I didn't even notice this change, but it looks obviously fine to me, with or without this patch. And yes, > Without the READ_ONCE(), the compiler might fuse reads from > consecutive calls to rcu_sync_is_idle() or (under register pressure) > re-read from it, getting inconsistent results. For example, this: > > tmp = rcu_sync_is_idle(rsp); > do_something(tmp); > do_something_else(tmp); > > Might become this: > > do_something(rcu_sync_is_idle(rsp)); > do_something_else(rcu_sync_is_idle(rsp)); this is very clear. Even for me ;) Thanks, Oleg.
