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List:       linux-kernel
Subject:    Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups
From:       "George Spelvin" <linux () horizon ! com>
Date:       2015-10-31 13:02:14
Message-ID: 20151031130214.10598.qmail () ns ! horizon ! com
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> 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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