Messages in this thread | | | Date | Tue, 6 Jul 2021 21:28:38 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH 2/2] rcu: Remove needless preemption disablement in rcu_all_qs() |
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On Tue, Jul 06, 2021 at 02:30:58PM +0200, Frederic Weisbecker wrote: > 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 think __this_cpu_write() being IRQ-unsafe means it may overwrite percpu writes to other bytes in the same word? Let's say the rcu_urgent_qs is the lowest byte in the word, the pseduo asm code of __this_cpu_write() may be:
__this_cpu_write(ptr, v): long tmp = *ptr; tmp &= ~(0xff); tmp |= v; *ptr = tmp;
and the following sequence introduces an overwrite:
__this_cpu_write(ptr, v): // v is 0, and *ptr is 1 long tmp = *ptr; // tmp is 1 <interrupted> this_cpu_write() // modify another byte of *ptr, make it // 0xff01 <ret from interrupt> tmp &= ~(0xff) // tmp is 0 tmp |=v; // tmp is 0 *ptr = tmp; // *ptr is 0, overwrite a percpu write on // another field.
I know that many archs have byte-wise store, so compilers don't really have the reason to generate code as above, but __this_cpu_write() is just a normal write, nothing prevents this from happenning, unless I'm missing something here?
Regards, Boqun
> > I'm not quite used to rcu_tasks. Paul?
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