Subject: Re: kernel/relay.c: a strange usage of delayed_work
From: Tom Zanussi <zanussi@us.ibm.com>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Karim Yaghmour <karim@opersys.com>, linux-kernel@vger.kernel.org
In-Reply-To: <20070503213835.GA243@tv-sign.ru>
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	 <20070427184258.62794053.akpm@linux-foundation.org>
	 <20070503213835.GA243@tv-sign.ru>
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Date: Fri, 04 May 2007 00:42:30 -0500
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On Fri, 2007-05-04 at 01:38 +0400, Oleg Nesterov wrote:
> relay_switch_subbuf() does schedule_delayed_work(&buf->wake_readers, 1),
> wakeup_readers() only does wake_up_interruptible() and nothing more.
> 
> Why can't we use a plain timer for this?
> 
> In any case, this "wake_up ->read_wait after a minimal possible delay"
> looks somewhat strange to me, could you explain? just curious.
> 

The reason it's done that way is that if the event that causes the
relay_switch_subbuf() happens to be an event logged from schedule(), and
we directly call wake_up_interruptible() at that point, we lock up the
machine because it ends up back in schedule().  Deferring it avoids the
problem.

I don't see any problem with using a plain timer instead - I'll work up
a patch to make that change.

Tom


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