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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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