Re: [PATCH v2 1/7] entry: add arch_in_rcu_eqs()

[Nicolas Saenz Julienne]

On Wed, 2022-01-19 at 10:58 +0000, Mark Rutland wrote:
> 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 with 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 is it necessary to enable
> interrupts around guest entry in the middle of a period where core KVM
> code has entered an EQS.
> 
> So that architectueres 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.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sven Schnelle <svens@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>

Thanks,

-- 
Nicolás Sáenz