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

Luis R. Rodriguez
2017-06-29T19:44:58Z

On Thu, Jun 29, 2017 at 09:40:15PM +0200, Luis R. Rodriguez wrote:
> On Thu, Jun 29, 2017 at 11:59:29AM -0700, Linus Torvalds wrote:
> > On Thu, Jun 29, 2017 at 11:33 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> > > On Thu, 29 Jun 2017, Linus Torvalds wrote:
> > >
> > >> 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.
> >
> > The *reason* they wake up only one seems to be that there really is
> > just one. It's some per-cpu idle thread for kvm, and for RCU it's the
> > RCU workqueue thread.
> >
> > So the queue literally looks suspiciously pointless.
> >
> > But I might be wrong, and there can actually be multiple entries.
>
> Since this swake_up() --> swake_up_all() reportedly *fixed* the one wake up
> issue it would seem this does queue [0]. That said, I don't see any simple tests
> tools/testing/selftests/swait but then again we don't have test for regular
> waits either...
>
> [0] https://bugzilla.kernel.org/show_bug.cgi?id=195477

I should also note that the swake_up_all() should have only helped in cases where
3 cards were used, as if only 2 were used that should have been covered by just
the swake_up(). Unless of course I hear otherwise by the reporter, Nicolas or
from Jakub.

Luis