From: Boqun Feng <boqun.feng@gmail.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
linux-rt-users@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Paul E. McKenney" <paulmck@kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Steven Price <steven.price@arm.com>,
Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
Date: Tue, 10 Aug 2021 10:42:21 +0800 [thread overview]
Message-ID: <YRHnjQqf28/uFq+Z@boqun-archlinux> (raw)
In-Reply-To: <20210807005807.1083943-3-valentin.schneider@arm.com>
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@arm.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
>
next prev parent reply other threads:[~2021-08-10 2:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-07 0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-07 0:58 ` [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-07 0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
2021-08-07 1:42 ` Mike Galbraith
2021-08-08 16:15 ` Valentin Schneider
2021-08-10 12:49 ` Boqun Feng
2021-08-10 13:04 ` Valentin Schneider
2021-08-10 2:42 ` Boqun Feng [this message]
2021-08-10 9:26 ` Valentin Schneider
2021-08-07 0:58 ` [PATCH v2 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-07 0:58 ` [PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
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