On Fri, 2010-05-28 at 01:44 +1000, Nick Piggin wrote: > > I think this will change the behavior of 'sync_supers()' too much. ATM, > > it makes only one SB pass, then sleeps, then another one, then sleeps. > > And we should probably preserve this behavior. So I'd rather make it: > > > > if (supers_dirty) > > bdi_arm_supers_timer(); > > set_current_state(TASK_INTERRUPTIBLE); > > schedule(); > > > > This way we will keep the behavior closer to the original. > > Well your previous code had the same issue (ie. it could loop again > in sync_supers). But fair point perhaps. I think no, it either armed the timer of went to sleep, but it does not matter much :-) > But we cannot do the above, because again the timer might go off > before we set current state. We'd lose the wakeup and never wake > up again. > > Putting it inside set_current_state() should be OK. I suppose. Oh, right! > > There is spin_lock(&sb_lock) in sync_supers(), so strictly speak this > > 'smp_mb()' is not needed if we move supers_dirty = 0 into > > 'sync_supers()' and add a comment that a mb is required, in case some > > one modifies the code later? > > > > Or this is not worth it? > > It's a bit tricky. spin_lock only gives an acquire barrier, which > prevents CPU executing instructions inside the critical section > before acquiring the lock. It actually allows stores to be deferred > from becoming visible to other CPUs until inside the critical section. > So the load of sb->s_dirty could indeed still happen before the > store is seen. > > Locks do allow you to avoid thinking about barriers, but *only* when > all memory accesses to all shared variables are inside the locks > (or when a section has just a single access, which by definition don't > need ordering with another access). Oh, ok. I need to read carefully Documentation/memory-barriers.txt. > > BTW, do you want me to keep you to be the patch author, add your > > signed-off-by and my original commit message? > > I'm not concerned. You contributed more to the idea+implementation, > so record yourself as author. Ok, but thank you a lot for helping! -- Best Regards, Artem Bityutskiy (Артём Битюцкий) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
