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

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Message-ID: <20151031130214.10598.qmail@ns.horizon.com>
Date:	31 Oct 2015 09:02:14 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	bigeasy@...utronix.de, tglx@...utronix.de
Cc:	akpm@...ux-foundation.org, dave@...olabs.net,
	linux-kernel@...r.kernel.org, linux@...izon.com,
	manfred@...orfullife.com, peterz@...radead.org
Subject: Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups

> 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.
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