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

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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



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