Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()

Hi,

On 10/08/21 10:42, Boqun Feng wrote:

Hi,

On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote:

quoted

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 <redacted>
---
 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()?

You're absolutely right.

migrate_disable() protects the thread against being migrated due to
hotplug, but pure CPU affinity doesn't at all. kthread_is_per_cpu() doesn't
work either, because parking is not the only approach to hotplug for those
(e.g. per-CPU workqueue threads unbind themselves on hotplug, as in your
example).

One could hold cpus_read_lock(), but I don't see much point here. So that
has to be 

  return !preemptible() || current->migration_disabled;

Thanks!

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