Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups

[Davidlohr Bueso <dave@stgolabs.net>]

On Sat, 31 Oct 2015, George Spelvin wrote:

>> Don't we need to keep that NULL init? I might be missing something.
>
>I wondered the same thing, but on reading it, the cleanup is that he's
>gotten rid of the need for the entire thing.  Previously, there was a
>mechanism for detecting "wakeup not quite finished" that used a NULL
>value, but it's no longer needed.
>
>The resultant busy-waiting on the part of the woken-up task was the
>entire problem this patch aims to fix.  So it gets rid of a whole lot
>of code and barriers.  And, as you noticed, the comments explaining them.
>
>As the old code explained, the issue is that a task may exit as
>soon as r_msg is set, so the wakeup procedure has to be:
>- Ensure r_msg is set to NULL (special-case flag)
>- Do the wake up
>- Set r_msg to the final value
>
>The woken-up task has to spin as long as r_msg is NULL.  Ick.

I agree this is a nice cleanup. It is similar to what we did
with posix mqueues, but this one gets rid of even more code afaict.

>However, a wake_q keeps a reference to a task, so exiting is
>not a danger.  As long as wake_q_add precedes setting r_msg,
>all is well.

Yes, and this confirms that we still rely on the implicit barrier
from the cmpxchg as tglx mentioned. As such, we also need to keep
the pairing when reading 'r_msg' in do_msgrcv(), instead of dropping
the comments.

Thanks,
Davidlohr