Messages in this thread | | | Date | Fri, 15 Jul 2022 17:34:08 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] printk: Make console tracepoint safe in NMI() context |
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On Fri 2022-07-15 08:10:00, Paul E. McKenney wrote: > On Fri, Jul 15, 2022 at 09:51:56AM -0400, Steven Rostedt wrote: > > On Fri, 15 Jul 2022 14:39:52 +0200 > > Marco Elver <elver@google.com> wrote: > > > > > Couldn't this just use rcu_is_watching()? > > > > > > | * rcu_is_watching - see if RCU thinks that the current CPU is not idle > > > > Maybe, but I was thinking that Petr had a way to hit the issue that we > > worry about. But since the non _rcuide() call requires rcu watching, > > prehaps that is better to use.
I actually saw the warning even with simple sysrq+l. I wonder why I have missed it during testing. It was probably well hidden within the other backtraces.
I was not aware that rcu_is_watching() and rcu_is_idle_cpu() did basically the same. I used rcu_is_idle_cpu() because of the "idle" in the name and the function description ;-)
> In case this helps... ;-) > > The rcu_is_watching() function is designed to be used from the current > CPU, so it dispenses with memory ordering. However, it explicitly > disables preemption in order to avoid weird preemption patterns. > > The formulation that Marco used is designed to be used from a remote > CPU, and so it includes explicit memory ordering that is not needed > in this case. But it does not disable preemption. > > So if preemption is enabled at that point in tracing, you really want > to be using rcu_is_watching().
rcu_is_watching() is the right variant then. I am going to send v2.
Thanks a lot for the detailed explanation.
Best Regards, Petr
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