entry: Add arch_in_rcu_eqs()

All architectures have an interruptible RCU extended quiescent state
(EQS) as part of their idle sequences, where interrupts can occur
without RCU watching. Entry code must account for this and wake RCU as
necessary; the common entry code deals with this in irqentry_enter() by
treating any interrupt from an idle thread as potentially having
occurred within an EQS and waking RCU for the duration of the interrupt
via rcu_irq_enter() .. rcu_irq_exit().

Some architectures may have other interruptible EQSs which require
similar treatment. For example, on s390 it is necessary to enable
interrupts around guest entry in the middle of a period where core KVM
code has entered an EQS.

So that architectures can wake RCU in these cases, this patch adds a
new arch_in_rcu_eqs() hook to the common entry code which is checked in
addition to the existing is_idle_thread() check, with RCU woken if
either returns true. A default implementation is provided which always
returns false, which suffices for most architectures.

As no architectures currently implement arch_in_rcu_eqs(), there should
be no functional change as a result of this patch alone. A subsequent
patch will add an s390 implementation to fix a latent bug with missing
RCU wakeups.

[ajd@linux.ibm.com: rebase, fix commit message]

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20250708092742.104309-2-ajd@linux.ibm.com


Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-ID: <20250708092742.104309-2-ajd@linux.ibm.com>