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@xxxxxxx>
---
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
