linux-kernel - Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

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Message-ID: <CAJhHMCC7_GLmBroGhKWMJJKuWqhojQGf=9So1_x4nb2adQvqFg@mail.gmail.com>
Date:	Wed, 23 Jul 2014 22:36:19 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

On Wed, Jul 23, 2014 at 8:26 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote:
>> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is
>> due to the condition having been met. This commit checks this return value
>> for a spurious wake up before calling rcu_gp_init().
>>
>> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
>
> How does this added check help?  I don't see that it does.  If the flag
> is set, we want to wake up.  If we get a spurious wakeup, but then the
> flag gets set before we actually wake up, we still want to wake up.
>

So I took a look at the docs again, and using the return value is the
recommended way to check for spurious wakeups.

The condition in wait_event_interruptible() is checked when the task
is woken up (either due to stray signals or explicitly) and it returns
true if condition evaluates to true.

In the current scenario, if we get a spurious wakeup, we take the
costly path of checking this condition again (with a barrier and lock)
before going back to wait.

The scenario of getting an actual wakeup after getting a spurious
wakeup exists even today, this is the window after detecting a
spurious wakeup and before going back to wait. I am not sure if using
the return value enlarges that window as we are going back to sleep
immediately.

Thoughts?

-- 
Pranith
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