linux-kernel - Re: [PATCH v2 2/4] sched: Introduce is_pcpu

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Date:   Tue, 10 Aug 2021 10:42:21 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        rcu@...r.kernel.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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        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 v2 2/4] sched: Introduce is_pcpu_safe()

Hi,

On Sat, Aug 07, 2021 at 01:58:05AM +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>
> ---
>  include/linux/sched.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index debc960f41e3..b77d65f677f6 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1715,6 +1715,16 @@ 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
> +}

I wonder whether the following can happen, say thread A is a worker
thread for CPU 1, so it has the flag PF_NO_SETAFFINITY set.

	{ percpu variable X on CPU 2 is initially 0 }

	thread A
	========

	<preemption enabled>
	if (is_pcpu_safe()) { // nr_cpus_allowed == 1, so return true.
		<preempted>
		<hot unplug CPU 1>
			unbinder_workers(1); // A->cpus_mask becomes cpu_possible_mask
		<back to run on CPU 2>
		__this_cpu_inc(X);
		  tmp = X; // tmp == 0
		  <preempted>
		  <in thread B>
		  this_cpu_inc(X); // X becomes 1
		  <back to run A on CPU 2>
		  X = tmp + 1; // race!
	}

if so, then is_percpu_thread() doesn't indicate is_pcpu_safe()?

Regards,
Boqun

> +
>  /* 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
>