Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote:
[..]
> > As such this patch looks correct to me, one thing I noticed is that
> > you can check rcu_is_watching() like the lockdep-enabled code does.
> > That will tell you also if a reader-section is possible because in
> > extended-quiescent-states, RCU readers should be non-existent or
> > that's a bug.
> >
> Please correct me if I am wrong, reading from the comment in
> kernel/rcu/update.c rcu_read_lock_held_common()
> ..
>   * The reason for this is that RCU ignores CPUs that are
>  * in such a section, considering these as in extended quiescent state,
>  * so such a CPU is effectively never in an RCU read-side critical section
>  * regardless of what RCU primitives it invokes.
> 
> It seems rcu will treat this as lock not held rather than a fact that
> lock is not held. Is my understanding correct?

If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you
mean it is not a fact for erofs?

> The reason I chose not to consult rcu_is_watching() in this version
> is because check "sleeping function called from invalid context"
> will still get triggered (please see kernel/sched/core.c __might_resched())
> as it does not consult rcu_is_watching() instead looks at
> rcu_preempt_depth() which will be non-zero if rcu_read_lock()
> was called (only when CONFIG_PREEMPT_RCU is enabled).

I am assuming you mean you would grab the mutex accidentally when in an RCU
reader, and might_sleep() presumably in the mutex internal code will scream?

I would expect in the erofs code that rcu_is_watching() should always return
true, so it should not effect the decision of whether to block or not. I am
suggesting add the check for rcu_is_watching() into the *held() functions for
completeness.

// will be if (!true) when RCU is actively watching the CPU for readers.
bool rcu_read_lock_any_held() {
	if (!rcu_is_watching())
		return false;
	// do the rest..
}

> > Could you also verify that this patch does not cause bloating of the
> > kernel if lockdep is disabled?
> >
> Sure, I will do the comparison and send the details.

Thanks! This is indeed an interesting usecase of grabbing mutex / blocking in
the reader.

thanks,

 - Joel