Messages in this thread | | | From | Pranith Kumar <> | Date | Wed, 23 Jul 2014 22:36:19 -0400 | Subject | Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value |
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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?
-- Pranith
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