linux-kernel - [Semaphore API] down_interruptible

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Message-ID: <557E9030.6080901@free.fr>
Date:	Mon, 15 Jun 2015 10:43:28 +0200
From:	Mason <slash.tmp@...e.fr>
To:	LKML <linux-kernel@...r.kernel.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [Semaphore API] down_interruptible_timeout

Hello,

The semaphore API provides several flavors of the down primitive:

  down, down_interruptible, down_killable, down_trylock, down_timeout

As far as I can tell, they all call __down_common (except down_trylock,
which returns 1 where the others would sleep).

I was looking for a version
1) with a timeout
2) that could be interrupted
e.g. down_interruptible_timeout, but it doesn't exist.

It seems

  __down_common(sem, TASK_INTERRUPTIBLE, timeout);

would work as expected, no?

Do you know why it is not offered?
(Maybe there is a better way to achieve the same thing?)

[POST SCRIPTUM EDIT]
I found this 2007 discussion:
http://thread.gmane.org/gmane.linux.kernel/498034

At the time, Andrew said
"Nobody else has needed to invent new locking infrastructure
to do such things and I'd prefer not to do so now."
I suppose this is still true :-)
[/EDIT]

My use-case is pretty simple:

A) process-context kernel thread fills a FIFO and calls down(&fifo_empty);
B) ISR handles the FIFO-empty interrupt with up(&fifo_empty);

However, in case something goes wrong and the interrupt never fires,
I don't want the process to be stuck in an uninterruptible sleep.

Perhaps I can set a tiny timeout (e.g. 10 µs) and not worry about
the interruptible part for such a small duration? (Hmm, __down_common
calls schedule_timeout, which is jiffies-based. I don't think there
is a hrtimers flavor. So µs timeouts would be off the table?)

Or I could use the interruptible version, and let the user kill the
operation if necessary.

I'd like to hear your comments and suggestions.

Regards.
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