Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
From: Boqun Feng <hidden>
Date: 2021-08-10 02:42:36
Also in:
linux-rt-users, lkml, rcu
Hi, On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote:
quoted hunk ↗ jump to hunk
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()?
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
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