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? > > * > > - * 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.) Thanx, Paul > thanks, > > - Joel > > > > * > > * Make notrace because it can be called by the internal functions of > > * ftrace, and making this notrace removes unnecessary recursion calls. >
