[dm-devel] [patch 2/4] swait: export the symbols __prepare_to_swait and __finish_swait

[Peter Zijlstra]

On Tue, May 22, 2018 at 02:52:54PM -0400, Mike Snitzer wrote:
> On Tue, May 22 2018 at  2:34am -0400,
> Christoph Hellwig <hch at infradead.org> wrote:

> > Please CC the author and maintainers of the swait code.
> > 
> > My impression is that this is the wrong thing to do.  The swait code
> > is supposed to be simple and self contained, and if you want to do
> > anything else use normal waitqueues.
> 
> You said the same thing last time around.  I've since cc'd Peter and
> Thomas and haven't heard back, see:
> https://www.redhat.com/archives/dm-devel/2018-May/msg00048.html

Yeah, sorry, got lost :/

> The entire point of exporting these symbols is to allow use of the
> "simple waitqueue" code to optimize -- without resorting to using normal
> waitqueues.

So I don't immediately object to exporting them; however I do share some
of hch's concerns. The reason swait exists is to be deterministic (for
RT) -- something that regular wait code cannot be.

And by (ab)using / exporting the wait internal lock you risk loosing
that. So I don't think the proposed usage is bad, it is possible to
create badness.

So if we're going to export them; someone needs to keep an eye on things
and ensure the lock isn't abused.