On Friday 26 March 2010, Jiri Slaby wrote: > On 03/25/2010 11:14 PM, Rafael J. Wysocki wrote: > > On Wednesday 24 March 2010, Jiri Slaby wrote: > >> On 03/24/2010 09:42 PM, Pavel Machek wrote: > >>>> + if (test_bit(TODO_CLOSED, to_do_flags)) > >>>> + return -EIO; > >>>> + > >>>> + to_do_buf = buf; > >>>> + wmb(); > >>>> + set_bit(TODO_WORK, to_do_flags); > >>>> + wake_up_interruptible(&to_do_wait); > >>> > >>> Uhuh, open-coded barriers... these need to be commented, and I guess > >>> you just should not play this kind of trickery. > >> > >> It's just to ensure the to_do_buf store is not reordered with the > >> set_bit. I wanted to avoid locks as too heavy tools here. > > > > No, please use them, at least in a prototype version. > > > > We can always optimize things out later, but doing optimizations upfront > > doesn't really work well from my experience. > > > > So, if you'd use a lock somewhere, please use it, or maybe use a completion if > > that fits the design better. > > That's it, I don't think a lock is appropriate here (I didn't even think > of that) -- I don't know what to lock (OK, I see it, but it's not that > clear). There is no potential for race per se, I only need to disable > reordering (which locks do as a side-effect). I need the steps to be > done in the A-B order where there is a barrier appropriate. Here, A is > store to to_do_buf, B is set_bit. It's I set to_do_buf, flag that it may > be used, the consumer will see the flag and use to_do_buf, in this order. > > Above that if I introduce locks the wait_event on the other side will > grow into an unreadable mess. I would need to hold a lock when checking > the condition and hold it until I reach to_do_buf use, but also unlock > it on all paths that do not reach that point. Yeah, it's indeed doable, > but I don't think, it will improve things. > > I also don't think completion is appropriate here, as I have a condition > to check for and it differs over wake_up sites. OK I have some other comments to this patch, but I'd like to understand what really happens here, since the changelog is not too verbose. Please explain the design here. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
