On Tue, 2012-10-09 at 06:37 -0700, Andi Kleen wrote: > Ivo Sieben <meltedpianoman@xxxxxxxxx> writes: > > > Check the waitqueue task list to be non empty before entering the critical > > section. This prevents locking the spin lock needlessly in case the queue > > was empty, and therefor also prevent scheduling overhead on a PREEMPT_RT > > system. > > > > Signed-off-by: Ivo Sieben <meltedpianoman@xxxxxxxxx> > > --- > > > > REPOST: > > Request for comments: > > - Does this make any sense? > > Looks good to me. Avoiding any spinlock is good. I don't even think you > need "careful", if you miss an update it was just as it happened a > little later. Yeah, so I've started looking at this several times, but never had the time to actually think it through. I think I'll agree with you that using the careful list empty check isn't needed. In general there's already the race of doing a wakeup before the wait and if the code using the waitqueue doesn't deal with that its already broken, so in that respect you should be good, since you're simply shifting the timing a little. One thing you might need to consider is the memory ordering, will the list_empty -- either careful or not -- observe the right list pointer, or could it -- when racing with wait_event()/prepare_to_wait() -- observe a stale value. Or.. is that all already covered in on the use site. I started auditing a few users to see what they all assume, if they're already broken and or if they would now be broken.. but like said, I didn't get anywhere. I'd like to see this patch/Changelog amended to at least cover these points so that I feel all warm and fuzzy when I read it and not have to think too hard ;-) -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
