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

From: Thomas Gleixner
Date: Sat Oct 31 2015 - 11:18:33 EST

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.
> 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.

Right. I figured that after staring some more into it. Though it would
be nice if exactly that explanation is in the code.



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