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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <laijs@cn.fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value
Date: Wed, 23 Jul 2014 20:43:15 -0700	[thread overview]
Message-ID: <20140724034315.GJ11241@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJhHMCC7_GLmBroGhKWMJJKuWqhojQGf=9So1_x4nb2adQvqFg@mail.gmail.com>

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@linux.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@gmail.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


  reply	other threads:[~2014-07-24  3:43 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-23  5:09 [PATCH 00/16] rcu: Some minor fixes and cleanups Pranith Kumar
2014-07-23  5:09 ` [PATCH 01/16] rcu: Use rcu_num_nodes instead of NUM_RCU_NODES Pranith Kumar
2014-07-23  5:09 ` [PATCH 02/16] rcu: Check return value for cpumask allocation Pranith Kumar
2014-07-23 12:06   ` Paul E. McKenney
2014-07-23 12:49     ` Pranith Kumar
2014-07-23 17:14     ` Pranith Kumar
2014-07-23 18:01       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 03/16] rcu: Fix comment for gp_state field values Pranith Kumar
2014-07-23  5:09 ` [PATCH 04/16] rcu: Remove redundant check for an online CPU Pranith Kumar
2014-07-23 12:09   ` Paul E. McKenney
2014-07-23 13:23     ` Pranith Kumar
2014-07-23 13:41       ` Paul E. McKenney
2014-07-23 14:01         ` Pranith Kumar
2014-07-23 14:14           ` Paul E. McKenney
2014-07-23 15:07             ` Pranith Kumar
2014-07-23 15:21               ` Pranith Kumar
2014-07-23  5:09 ` [PATCH 05/16] rcu: Add noreturn attribute to boost kthread Pranith Kumar
2014-07-23  5:09 ` [PATCH 06/16] rcu: Clear gp_flags only when actually starting new gp Pranith Kumar
2014-07-23 12:13   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 07/16] rcu: Save and restore irq flags in rcu_gp_cleanup() Pranith Kumar
2014-07-23 12:16   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 08/16] rcu: Clean up rcu_spawn_one_boost_kthread() Pranith Kumar
2014-07-23  5:09 ` [PATCH 09/16] rcu: Remove redundant check for online cpu Pranith Kumar
2014-07-23 12:21   ` Paul E. McKenney
2014-07-23 12:59     ` Pranith Kumar
2014-07-23 13:50       ` Paul E. McKenney
2014-07-23 14:12         ` Pranith Kumar
2014-07-23 14:23           ` Paul E. McKenney
2014-07-23 15:11             ` Pranith Kumar
2014-07-23 15:30               ` Paul E. McKenney
2014-07-23 15:44                 ` Pranith Kumar
2014-07-23 19:15                   ` Paul E. McKenney
2014-07-23 20:01                     ` Pranith Kumar
2014-07-23 20:16                     ` Pranith Kumar
2014-07-23 20:23                       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 10/16] rcu: Check for RCU_FLAG_GP_INIT bit in gp_flags for spurious wakeup Pranith Kumar
2014-07-23 12:23   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 11/16] rcu: Check for spurious wakeup using return value Pranith Kumar
2014-07-23 12:26   ` Paul E. McKenney
2014-07-24  2:36     ` Pranith Kumar
2014-07-24  3:43       ` Paul E. McKenney [this message]
2014-07-24  4:03         ` Pranith Kumar
2014-07-24 18:12           ` Paul E. McKenney
2014-07-24 19:59             ` Pranith Kumar
2014-07-24 20:27               ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 12/16] rcu: Rename rcu_spawn_gp_kthread() to rcu_spawn_kthreads() Pranith Kumar
2014-07-23  5:09 ` [PATCH 13/16] rcu: Spawn nocb kthreads from rcu_prepare_kthreads() Pranith Kumar
2014-07-23  5:09 ` [PATCH 14/16] rcu: Remove redundant checks for rcu_scheduler_fully_active Pranith Kumar
2014-07-23 12:27   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 15/16] rcu: Check for a nocb cpu before trying to spawn nocb threads Pranith Kumar
2014-07-23 12:28   ` Paul E. McKenney
2014-07-23 13:14     ` Pranith Kumar
2014-07-23 13:42       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 16/16] rcu: kvm.sh: Fix error when you pass --cpus argument Pranith Kumar
2014-07-23 12:31   ` Paul E. McKenney
2014-07-23 14:45 ` [PATCH 00/16] rcu: Some minor fixes and cleanups Paul E. McKenney
2014-08-27  1:10   ` Pranith Kumar
2014-08-27  3:20     ` Paul E. McKenney

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