linux-kernel - Re: [PATCH 1/3] sched: Introduce is_pcpu

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Message-ID: <20210727162327.GA4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 27 Jul 2021 09:23:27 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rt-users@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Steven Price <steven.price@....com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 1/3] sched: Introduce is_pcpu_safe()

On Wed, Jul 21, 2021 at 12:51:16PM +0100, Valentin Schneider wrote:
> Some areas use preempt_disable() + preempt_enable() to safely access
> per-CPU data. The PREEMPT_RT folks have shown this can also be done by
> keeping preemption enabled and instead disabling migration (and acquiring a
> sleepable lock, if relevant).
> 
> Introduce a helper which checks whether the current task can safely access
> per-CPU data, IOW if the task's context guarantees the accesses will target
> a single CPU. This accounts for preemption, CPU affinity, and migrate
> disable - note that the CPU affinity check also mandates the presence of
> PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
> upcoming per-CPU access(es) unsafe.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  include/linux/sched.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index efdbdf654876..7ce2d5c1ad55 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1707,6 +1707,16 @@ static inline bool is_percpu_thread(void)
>  #endif
>  }
>  
> +/* Is the current task guaranteed not to be migrated elsewhere? */
> +static inline bool is_pcpu_safe(void)
> +{
> +#ifdef CONFIG_SMP
> +	return !preemptible() || is_percpu_thread() || current->migration_disabled;
> +#else
> +	return true;
> +#endif
> +}
> +
>  /* Per-process atomic flags. */
>  #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
>  #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
> -- 
> 2.25.1
>