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

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Date:	Sat, 31 Oct 2015 16:17:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	George Spelvin <linux@...izon.com>
cc:	bigeasy@...utronix.de, akpm@...ux-foundation.org,
	dave@...olabs.net, linux-kernel@...r.kernel.org,
	manfred@...orfullife.com, peterz@...radead.org
Subject: Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups

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.

Thanks,

	tglx

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