On Thu, 6 Aug 2015 15:22:45 -0400
Paul Gortmaker [off-list ref] wrote:

[[RFC v0 0/3] Simple wait queue support] On 05/08/2015 (Wed 15:30) Daniel Wagner wrote:

quoted

Hi,

It's a while since the last attempt by Paul to get simple wait ready
for mainline [1]. At the last realtime workshop it was discussed how
the swait implementation could be made preempt aware. Peter posted an
untested version of it here [2].

So, from memory, here are the issues or questions that need answers
before we can consider trying mainline IMO.

1) naming: do we keep the swait, do we try and morph complex wait users
   into using cwait, or some mix of the two, or ... ?

I would say keep swait for now. Convert a few major hitters to it to
test it out. After a release or two, create a cwait, and start
converting the complex waits to them. Then after a period of stability,
convert the normal wait_queues to be implemented with swait, and remove
the swait from the kernel.

2) placement: as I think I said before, the standalone files works for
   the -rt patches because it is the lowest maintenance solution, but
   IMO for mainline, the simple and complex versions should be right
   beside each other so they can be easily contrasted and compared and
   so any changes to one will naturally also flow to the other.

I'm agnostic on this part.

3) barrier usage:  we'd had some questions and patches in the past that
   futz'd around with the use of barriers, and as a mainline requirement
   we'd need someone to check, understand and document them all properly.

Sounds like a plan.

4) poll_wait: currently it and poll_table_entry are both hard coupled
   to wait_queue_head_t -- so any users of poll_wait are not eligible
   for conversion to simple wait. (I just happened to notice that
   recently.)  A quick grep shows ~500 poll_wait users.

Looks like a good candidate to test the cwait on :-)

5) the aforementioned "don't do an unbounded number of callbacks while
   holding the raw lock" issue.

We should solve #5 for -rt regardless; I wouldn't attempt to make a
new "for mainline" set again w/o some consensus on #1 and #2, and I
think it would take someone like peterz/paulmck/rostedt to do #3
properly.  I don't know if #4 is an issue we need to worry about
right away; probably not.  And I'm sure I'll think of some other
issue five seconds after I hit send...

5... 4... 3... 2... 1... (where's the other issue?)

-- Steve