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

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Date:	Wed, 23 Jul 2014 20:43:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.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 10:36:19PM -0400, Pranith Kumar wrote:
> 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?

If the flag is set, why should we care whether or not the wakeup was
spurious?  If the flag is not set, why should we care whether or not
wait_event_interruptible() thought that the wakeup was not spurious?

							Thanx, Paul

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