On Wed, Jul 23, 2014 at 8:26 AM, Paul E. McKenney <paul...@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.pr...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
