On Tue, Jul 18, 2023 at 09:56:38PM -0400, Joel Fernandes wrote: > Hi Paul, > > On 7/18/23 14:12, Paul E. McKenney wrote: > > On Tue, Jul 18, 2023 at 08:52:30AM -0400, Joel Fernandes wrote: > > > Hi Paul, > > > > > > On 7/17/23 14:03, Paul E. McKenney wrote: > > > > Make it clear that this function always returns either true or false > > > > without other planned failure modes. > > > > > > > > Reported-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > --- > > > > kernel/rcu/tree.c | 12 ++++++++---- > > > > 1 file changed, 8 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index 1449cb69a0e0..fae9b4e29c93 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -679,10 +679,14 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp) > > > > /** > > > > * rcu_is_watching - see if RCU thinks that the current CPU is not idle > > > > > > Would it be better to modify the 'not idle' to 'not idle from an RCU > > > viewpoint'? This matches the comments in ct_nmi_enter() as well. > > > > We have the "if RCU thinks that" earlier. > > > > But maybe something like this? > > > > * rcu_is_watching - RCU read-side critical sections permitted on current CPU? > > > > Yes, that's better. > > > > > * > > > > - * Return true if RCU is watching the running CPU, which means that this > > > > - * CPU can safely enter RCU read-side critical sections. In other words, > > > > - * if the current CPU is not in its idle loop or is in an interrupt or > > > > - * NMI handler, return true. > > > > + * Return @true if RCU is watching the running CPU and @false otherwise. > > > > + * An @true return means that this CPU can safely enter RCU read-side > > > > + * critical sections. > > > > + * > > > > + * More specifically, if the current CPU is not deep within its idle > > > > + * loop, return @true. Note that rcu_is_watching() will return @true if > > > > + * invoked from an interrupt or NMI handler, even if that interrupt or > > > > + * NMI interrupted the CPU while it was deep within its idle loop. > > > > > > But it is more than the idle loop, for ex. NOHZ_FULL CPUs with single task > > > running could be idle from RCU's viewpoint? Could that be clarified more? > > > > Perhaps something like this? > > > > * Although calls to rcu_is_watching() from most parts of the kernel > > * will return @true, there are important exceptions. For example, if the > > * current CPU is deep within its idle loop, in kernel entry/exit code, > > * or offline, rcu_is_watching() will return @false. > > > > (Where nohz_full CPUs are covered by kernel entry/exit code.) > > To me, "kernel exit" does not immediately make the nohz_full CPU case > obvious. But yes, your suggestion is an improvement so we can go with that. > :) > > Also because we agree on the changes, for next revision: > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Very good, thank you! The update should appear shortly. For some definition of "shortly". ;-) Thanx, Paul
