Re: [PATCH 2/4] swait: add the missing killable swaits

[Davidlohr Bueso]

On Thu, 29 Jun 2017, Linus Torvalds wrote:

> So without some very compelling reason, I'd not want to add yet
> another wait-queue.

Yes, I was expecting this and very much agree.

I'll actually take a look at wake_q for wake_up_all() and co. to see if
we can reduce the spinlock hold times. Of course it would only make sense
for more than a one wakeup.

> I actually think swait is pure garbage. Most users only wake up one
> process anyway, and using swait for that is stupid. If you only wake
> up one, you might as well just have a single process pointer, not a
> wait list at all, and then use "wake_up_process()".

But you still need the notion of a queue, even if you wake one task
at a time... I'm probably missing your point here.

> There is *one* single user of swake_up_all(), and that one looks like
> bogus crap also: it does it outside of the spinlock that could have
> been used to protect the queue - p,lus I'm not sure there's really a
> queue anyway, since I think it's just the grace-period kthread that is
> there.

So those cases when there's only one waiter I completely agree should
not be using waitqueues. pcpu-rwsems in the past suffered from this.

Thanks,
Davidlohr