Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

Frederic Weisbecker
2021-09-30T10:53:44Z

On Thu, Sep 30, 2021 at 10:00:39AM +0100, Valentin Schneider wrote:
> Hi,
>
> On 21/09/21 23:12, 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.
> >
>
> Thanks for taking a look at this!
>
> My reasoning for adding protection in the outer functions was to prevent
> impaired unlocks of rcu_nocb_{un}lock_irqsave(), as that too depends on the
> offload state. Cf. Frederic's writeup:
>
> http://lore.kernel.org/r/20210727230814.GC283787@lothringen

I was wrong about that BTW!
Because rcu_nocb_lock() always require IRQs to be disabled, which of course disables
preemption, so the offloaded state can't change between
rcu_nocb_lock[_irqsave]() and rcu_nocb_unlock[_irqrestore]() but anyway there
were many other issues to fix :-)

>
> Anywho, I see Frederic has sent a fancy new series, let me go stare at it.