Re: [RFC PATCH 07/18] kthread: Make iterant kthreads freezable by default

On Sat 2015-06-13 18:22:22, Tejun Heo wrote:

quoted

I try to better understand why freezer is considered to be a blunt
tool. Is it because it is a generic API, try_to_freeze() is put on
"random" locations, so that it does not define the safe point
precisely enough?

Not that.  I don't know how to explain it better.  Hmmm... okay, let's
say there's a shared queue Q and N users o fit.  If you wanna make Q
empty and keep it that way for a while, the right thing to do is
blocking new queueing and then wait till Q drains - you choke the
entity that you wanna control.

Instead of that, freezer is trying to block the "N" users part.  In
majority of cases, it blocks enough but it's pretty difficult to be
sure whether you actually got all N of them (as some of them may not
involve kthreads at all or unfreezable kthreads might end up doing
those operations too on corner cases) and it's also not that clear
whether blocking the N users actually make Q empty.  Maybe there are
things which can be in flight asynchronously on Q even all its N users
are blocked.  This is inherently finicky.

I feel convinced that it does not make sense to make kthreads
freezable by default and that we should not use it when not
necessary.

Thanks a lot for patience and so detailed explanation.

Best Regards,
Petr