Messages in this thread | | | Date | Tue, 6 Jul 2021 14:30:58 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/2] rcu: Remove needless preemption disablement in rcu_all_qs() |
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On Tue, Jul 06, 2021 at 09:51:01AM +0200, Peter Zijlstra wrote: > On Tue, Jul 06, 2021 at 01:43:44AM +0200, Frederic Weisbecker wrote: > > The preemption is already disabled when we write rcu_data.rcu_urgent_qs. > > We can use __this_cpu_write() directly, although that path is mostly > > used when CONFIG_PREEMPT=n. > > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > > Cc: Neeraj Upadhyay <neeraju@codeaurora.org> > > Cc: Joel Fernandes <joel@joelfernandes.org> > > Cc: Uladzislau Rezki <urezki@gmail.com> > > Cc: Boqun Feng <boqun.feng@gmail.com> > > --- > > kernel/rcu/tree_plugin.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 27b74352cccf..38b3d01424d7 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -871,7 +871,7 @@ void rcu_all_qs(void) > > preempt_enable(); > > return; > > } > > - this_cpu_write(rcu_data.rcu_urgent_qs, false); > > + __this_cpu_write(rcu_data.rcu_urgent_qs, false); > > There's another subtle difference between this_cpu_write() and > __this_cpu_write() aside from preempt. this_cpu_write() is also > IRQ-safe, while __this_cpu_write() is not. > > I've not looked at the usage here to see if that is relevant, but the > Changelog only mentioned the preempt side of things, and that argument > is incomplete in general.
You're right, I missed that. I see this rcu_urgent_qs is set by RCU TASKS from rcu_tasks_wait_gp() (did I missed another path?). Not sure if this is called from IRQ nor if it actually matters to protect against IRQs for that single write.
I'm not quite used to rcu_tasks. Paul?
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