linux-kernel - Re: [patch 2/4] new timerfd API v2

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Date:	Wed, 26 Sep 2007 09:14:14 +0200
From:	Michael Kerrisk <mtk-manpages@....net>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/4] new timerfd API v2 - new timerfd API



Davide Libenzi wrote:
> On Tue, 25 Sep 2007, Jonathan Corbet wrote:
> 
>> One quick question:
>>
>>> Like the previous timerfd API implementation, read(2) and poll(2) are supported
>>> (with the same interface).
>> Looking at that interface, it appears that a process doing a read() on a
>> timerfd with no timer set will block for a very long time.  It's an
>> obvious "don't do that" situation, but perhaps we could help an
>> occasional developer get a clue by returning something like -EINVAL when
>> the timer has not been set?
> 
> That is the same as you try to read once more after an expired timer. You 
> won't wake up until the next timer event will show up. That is, after at 
> most TP time for periodic timers, or after the time  the next 
> timerfd_settime() will setup.
> I'd like to keep the "timerfd not set yet" and the "timerfd already 
> expired and not re-armed" acting the same way. That is, wait till next 
> event happen (unless O_NONBLOCK of course).

Yes.  The timer_settime() and read() might for example be done in separate
threads, and it would make sense for the read() to block until the timer
has been armed.

Cheers,

Michael


-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7

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