Re: [PATCH 11/15] tick: Move got_idle_tick away from common flags

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Jan 24 2024 at 18:04, Frederic Weisbecker wrote:
> tick_nohz_idle_got_tick() is called by cpuidle_reflect() within the idle
> loop with interrupts enabled. This function modifies the struct
> tick_sched's bitfield "got_idle_tick". However this bitfield is stored
> within the same mask as other bitfields that can be modified from
> interrupts.
>
> Fortunately so far it looks like the only race that can happen is while
> writing ->got_idle_tick to 0, an interrupt fires and writes the
> ->idle_active field to 0. It's then possible that the interrupted write
> to ->got_idle_tick writes back the old value of ->idle_active back to 1.
>
> However if that happens, the worst possible outcome is that the time
> spent between that interrupt and the upcoming call to
> tick_nohz_idle_exit() is accounted as idle, which is negligible quantity.
>
> Still all the bitfield writes within this struct tick_sched's shadow
> mask should be IRQ-safe. Therefore move this bitfield out to its own
> storage to avoid further suprises.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>