Re: [RFC] introduce prepare_to_wait_event()

[Oleg Nesterov <oleg@redhat.com>]

On 10/01, Peter Zijlstra wrote:
>
> On Tue, Oct 01, 2013 at 07:01:37PM +0200, Oleg Nesterov wrote:
> > 		if (exclusive)							\
> > 			__wait.flags = WQ_FLAG_EXCLUSIVE;			\
> > 		else								\
> > 			__wait.flags = 0;					\
>
> 		__wait.flags = exclusive * WQ_FLAG_EXCLUSIVE;
>
> or is that too obscure? ;-)

I do not mind ;) The generated code should be the same.

> > 		for (;;) {							\
> > 			long intr = prepare_to_wait_event(&wq, &__wait, state);	\
>
> 			int __intr = ...;
>
> The interruptible bit doesn't actually need long;

Yes, it can be even "bool", but see below.

> and local variables
> have __ prefixes in this context.

Yes, yes, will fix.

> > 			if (condition)						\
> > 				break;						\
> > 										\
> > 			if (___wait_is_interruptible(state) && intr) {		\
> > 				__ret = intr;					\

Since typeof(__ret) == typeof(intr) gcc can (likely) simply do "mov r1, r2",
so "long intr" make the code better.

I am not saying that "int intr" can make it worse, but to me "long" looks
better in this context. But I wouldn't mind to change this.

> > Compiler should optimize out "long intr" if !interruptible/killable.
>
> Yeah, and I think even the if (0 && __intr) would suffice for the unused
> check; otherwise we'd have to adorn the thing with __maybe_unused.

Hmm yes, I didn't see any warning during the compilation, but perhaps
__maybe_unused is needed, thanks.

> > What do you think?
>
> That would actually work I think.. the ___wait_is_interruptible() nicely
> does away with the unused code; the only slightly more expensive thing
> would be the prepare_to_wait_event() thing.
>
> And if that really turns out to be a problem we could even re-use
> ___wait_is_interruptible() to call prepare_to_wait() instead.

OK, thanks.

So I'll wait until your series is applied the resend it officially.

Oleg.