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. thanks, -- js _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
