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From: Thomas Gleixner <tglx@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>, LKML
 <linux-kernel@vger.kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>, Ingo Molnar
 <mingo@kernel.org>, Anna-Maria Behnsen <anna-maria@linutronix.de>, Peng
 Liu <liupeng17@lenovo.com>, Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 11/15] tick: Move got_idle_tick away from common flags
In-Reply-To: <20240124170459.24850-12-frederic@kernel.org>
References: <20240124170459.24850-1-frederic@kernel.org>
 <20240124170459.24850-12-frederic@kernel.org>
Date: Thu, 25 Jan 2024 10:40:24 +0100
Message-ID: <87o7d9iwdz.ffs@tglx>
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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>