>
> Hrm, rcu_read_unlock_sched_notrace() can still call
> __preempt_schedule_notrace(), which is again instrumented by KMSAN.
>
> This patch gets me a working kernel:
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 4ed33b127821..2d62df462d88 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2000,6 +2000,7 @@ static inline int pfn_valid(unsigned long pfn)
> {
> struct mem_section *ms;
> int ret;
> + unsigned long flags;
>
> /*
> * Ensure the upper PAGE_SHIFT bits are clear in the
> @@ -2013,9 +2014,9 @@ static inline int pfn_valid(unsigned long pfn)
> if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> return 0;
> ms = __pfn_to_section(pfn);
> - rcu_read_lock();
> + local_irq_save(flags);
> if (!valid_section(ms)) {
> - rcu_read_unlock();
> + local_irq_restore(flags);
> return 0;
> }
> /*
> @@ -2023,7 +2024,7 @@ static inline int pfn_valid(unsigned long pfn)
> * the entire section-sized span.
> */
> ret = early_section(ms) || pfn_section_valid(ms, pfn);
> - rcu_read_unlock();
> + local_irq_restore(flags);
>
> return ret;
> }
>
> Disabling interrupts is a little heavy handed - it also assumes the
> current RCU implementation. There is
> preempt_enable_no_resched_notrace(), but that might be worse because it
> breaks scheduling guarantees.
>
> That being said, whatever we do here should be wrapped in some
> rcu_read_lock/unlock_<newvariant>() helper.
We could as well redefine rcu_read_lock/unlock in mm/kmsan/shadow.c
(or the x86-specific KMSAN header, depending on whether people are
seeing the problem on s390 and Power) with some header magic.
But that's probably more fragile than adding a helper.
>
> Is there an existing helper we can use? If not, we need a variant that
> can be used from extremely constrained contexts that can't even call
> into the scheduler. And if we want pfn_valid() to switch to it, it also
> should be fast.
