rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

[Paul E. McKenney]

On Wed, Sep 22, 2021 at 01:36:27AM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 21, 2021 at 11:12:50PM +0200, Thomas Gleixner wrote:
> > Valentin reported warnings about suspicious RCU usage on RT kernels. Those
> > happen when offloading of RCU callbacks is enabled:
> > 
> >   WARNING: suspicious RCU usage
> >   5.13.0-rt1 #20 Not tainted
> >   -----------------------------
> >   kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state!
> > 
> >   rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58)
> >   rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777)
> >   rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876)
> > 
> > The reason is that rcu_rdp_is_offloaded() is invoked without one of the
> > required protections on RT enabled kernels because local_bh_disable() does
> > not disable preemption on RT.
> > 
> > Valentin proposed to add a local lock to the code in question, but that's
> > suboptimal in several aspects:
> > 
> >   1) local locks add extra code to !RT kernels for no value.
> > 
> >   2) All possible callsites have to audited and amended when affected
> >      possible at an outer function level due to lock nesting issues.
> > 
> >   3) As the local lock has to be taken at the outer functions it's required
> >      to release and reacquire them in the inner code sections which might
> >      voluntary schedule, e.g. rcu_do_batch().
> > 
> > Both callsites of rcu_rdp_is_offloaded() which trigger this check invoke
> > rcu_rdp_is_offloaded() in the variable declaration section right at the top
> > of the functions. But the actual usage of the result is either within a
> > section which provides the required protections or after such a section.
> > 
> > So the obvious solution is to move the invocation into the code sections
> > which provide the proper protections, which solves the problem for RT and
> > does not have any impact on !RT kernels.
> 
> You also need to consider rcu_segcblist_completely_offloaded(). There
> are two users:
> 
> 1) The first chunk using it in rcu_core() checks if there is a need to
> accelerate the callback and that can happen concurrently with nocb
> manipulations on the cblist. Concurrent (de-)offloading could mess
> up with the locking state but here is what we can do:
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bce848e50512..3e56a1a4af03 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2728,9 +2728,10 @@ static __latent_entropy void rcu_core(void)
>  
>  	/* No grace period and unregistered callbacks? */
>  	if (!rcu_gp_in_progress() &&
> -	    rcu_segcblist_is_enabled(&rdp->cblist) && do_batch) {
> +	    rcu_segcblist_is_enabled(&rdp->cblist)) {
>  		rcu_nocb_lock_irqsave(rdp, flags);
> -		if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
> +		if (!rcu_segcblist_completely_offloaded(&rdp->cblist) &&
> +		    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
>  			rcu_accelerate_cbs_unlocked(rnp, rdp);
>  		rcu_nocb_unlock_irqrestore(rdp, flags);
>  	}
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 305cf6aeb408..64d615be3346 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -449,10 +449,9 @@ static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp);
>  static void __init rcu_organize_nocb_kthreads(void);
>  #define rcu_nocb_lock_irqsave(rdp, flags)				\
>  do {									\
> +	local_irq_save(flags);						\
>  	if (!rcu_segcblist_is_offloaded(&(rdp)->cblist))		\
> -		local_irq_save(flags);					\
> -	else								\
> -		raw_spin_lock_irqsave(&(rdp)->nocb_lock, (flags));	\
> +		raw_spin_lock(&(rdp)->nocb_lock);			\
>  } while (0)
>  #else /* #ifdef CONFIG_RCU_NOCB_CPU */
>  #define rcu_nocb_lock_irqsave(rdp, flags) local_irq_save(flags)
> 
> 
> Doing the local_irq_save() before checking that the segcblist is offloaded
> protect that state from being changed (provided we lock the local rdp). Then we
> can safely manipulate cblist, whether locked or unlocked.
> 
> 2) The actual call to rcu_do_batch(). If we are preempted between
> rcu_segcblist_completely_offloaded() and rcu_do_batch() with a deoffload in
> the middle, we miss the callback invocation. Invoking rcu_core by the end of
> deoffloading process should solve that.

Maybe invoke rcu_core() at that point?  My concern is that there might
be an extended time between the missed rcu_do_batch() and the end of
the deoffloading process.

> > Reported-by: Valentin Schneider <valentin.schneider at arm.com>
> > Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> > ---
> >  kernel/rcu/tree.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2278,13 +2278,13 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> >  {
> >  	unsigned long flags;
> >  	unsigned long mask;
> > -	bool needwake = false;
> > -	const bool offloaded = rcu_rdp_is_offloaded(rdp);
> > +	bool offloaded, needwake = false;
> >  	struct rcu_node *rnp;
> >  
> >  	WARN_ON_ONCE(rdp->cpu != smp_processor_id());
> >  	rnp = rdp->mynode;
> >  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > +	offloaded = rcu_rdp_is_offloaded(rdp);
> >  	if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
> >  	    rdp->gpwrap) {
> 
> BTW Paul, if we happen to switch to non-NOCB (deoffload) some time after
> rcu_report_qs_rdp(), it's possible that the rcu_accelerate_cbs()
> that was supposed to be handled by nocb kthreads on behalf of
> rcu_core() -> rcu_report_qs_rdp() would not happen. At least not until
> we invoke rcu_core() again. Not sure how much harm that could cause.

Again, can we just invoke rcu_core() as soon as this is noticed?

> > @@ -2446,7 +2446,7 @@ static void rcu_do_batch(struct rcu_data
> >  	int div;
> >  	bool __maybe_unused empty;
> >  	unsigned long flags;
> > -	const bool offloaded = rcu_rdp_is_offloaded(rdp);
> > +	bool offloaded;
> >  	struct rcu_head *rhp;
> >  	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
> >  	long bl, count = 0;
> > @@ -2472,6 +2472,7 @@ static void rcu_do_batch(struct rcu_data
> >  	rcu_nocb_lock(rdp);
> >  	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
> >  	pending = rcu_segcblist_n_cbs(&rdp->cblist);
> > +	offloaded = rcu_rdp_is_offloaded(rdp);
> 
> offloaded is also checked later in rcu_do_batch(), after irqrestore. In
> fact that should even become a rcu_segcblist_completely_offloaded() check
> there because if there are still pending callbacks while we are de-offloading,
> rcu_core() should be invoked. Hmm but that might be a remote rcu_core...
> 
> Anyway I guess we could live with some of those races with invoking rcu core on the
> target after deoffloading.
> 
> I guess I should cook a series to handle all these issues one by one, then
> probably we can live without a local_lock().

And thank you very much for looking this over!  Not simple stuff.  ;-)

							Thanx, Paul

> Thanks.
> 
> 
> 
> >  	div = READ_ONCE(rcu_divisor);
> >  	div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
> >  	bl = max(rdp->blimit, pending >> div);