Add a debugging infrastructure to warn if an atomic hotplug reader has not
invoked get_online_cpus_atomic() before traversing/accessing the
cpu_online_mask. Encapsulate these checks under a new debug config option
DEBUG_HOTPLUG_CPU.
This debugging infrastructure proves useful in the tree-wide conversion
of atomic hotplug readers from preempt_disable() to the new APIs, and
help us catch the places we missed, much before we actually get rid of
stop_machine(). We can perhaps remove the debugging checks later on.
Cc: Rusty Russell <ru...@rustcorp.com.au>
Cc: Alex Shi <alex....@intel.com>
Cc: KOSAKI Motohiro <kosaki.motoh...@jp.fujitsu.com>
Cc: Tejun Heo <t...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasu...@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wyso...@intel.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
---
include/linux/cpumask.h | 12 ++++++
kernel/cpu.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 101 insertions(+)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index d08e4d2..9197ca4 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -101,6 +101,18 @@ extern const struct cpumask *const cpu_active_mask;
#define cpu_active(cpu) ((cpu) == 0)
#endif
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU
+extern void check_hotplug_safe_cpumask(const struct cpumask *mask);
+extern void check_hotplug_safe_cpu(unsigned int cpu,
+ const struct cpumask *mask);
+#else
+static inline void check_hotplug_safe_cpumask(const struct cpumask *mask) { }
+static inline void check_hotplug_safe_cpu(unsigned int cpu,
+ const struct cpumask *mask)
+{
+}
+#endif
+
/* verify cpu argument to cpumask_* operators */
static inline unsigned int cpumask_check(unsigned int cpu)
{
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 860f51a..5297ec1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -63,6 +63,92 @@ static struct {
.refcount = 0,
};
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU
+
+static DEFINE_PER_CPU(unsigned long, atomic_reader_refcnt);
+
+static int current_is_hotplug_safe(const struct cpumask *mask)
+{
+
+ /* If we are not dealing with cpu_online_mask, don't complain. */
+ if (mask != cpu_online_mask)
+ return 1;
+
+ /* If this is the task doing hotplug, don't complain. */
+ if (unlikely(current == cpu_hotplug.active_writer))
+ return 1;
+
+ /* If we are in early boot, don't complain. */
+ if (system_state != SYSTEM_RUNNING)
+ return 1;
+
+ /*
+ * Check if the current task is in atomic context and it has
+ * invoked get_online_cpus_atomic() to synchronize with
+ * CPU Hotplug.
+ */
+ if (preempt_count() || irqs_disabled())
+ return this_cpu_read(atomic_reader_refcnt);
+ else
+ return 1; /* No checks for non-atomic contexts for now */
+}
+
+static inline void warn_hotplug_unsafe(void)
+{
+ WARN_ONCE(1, "Must use get/put_online_cpus_atomic() to synchronize"
+ " with CPU hotplug\n");
+}
+
+/*
+ * Check if the task (executing in atomic context) has the required protection
+ * against CPU hotplug, while accessing the specified cpumask.
+ */
+void check_hotplug_safe_cpumask(const struct cpumask *mask)
+{
+ if (!current_is_hotplug_safe(mask))
+ warn_hotplug_unsafe();
+}
+EXPORT_SYMBOL_GPL(check_hotplug_safe_cpumask);
+
+/*
+ * Similar to check_hotplug_safe_cpumask(), except that we don't complain
+ * if the task (executing in atomic context) is testing whether the CPU it
+ * is executing on is online or not.
+ *
+ * (A task executing with preemption disabled on a CPU, automatically prevents
+ * offlining that CPU, irrespective of the actual implementation of CPU
+ * offline. So we don't enforce holding of get_online_cpus_atomic() for that
+ * case).
+ */
+void check_hotplug_safe_cpu(unsigned int cpu, const struct cpumask *mask)
+{
+ if(!current_is_hotplug_safe(mask) && cpu != smp_processor_id())
+ warn_hotplug_unsafe();
+}
+EXPORT_SYMBOL_GPL(check_hotplug_safe_cpu);
+
+static inline void atomic_reader_refcnt_inc(void)
+{
+ this_cpu_inc(atomic_reader_refcnt);
+}
+
+static inline void atomic_reader_refcnt_dec(void)
+{
+ this_cpu_dec(atomic_reader_refcnt);
+}
+
+#else
+
+static inline void atomic_reader_refcnt_inc(void)
+{
+}
+
+static inline void atomic_reader_refcnt_dec(void)
+{
+}
+
+#endif
+
void get_online_cpus(void)
{
might_sleep();
@@ -189,12 +275,15 @@ unsigned int get_online_cpus_atomic(void)
* from going offline.
*/
preempt_disable();
+ atomic_reader_refcnt_inc();
+
return smp_processor_id();
}
EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
void put_online_cpus_atomic(void)
{
+ atomic_reader_refcnt_dec();
preempt_enable();
}
EXPORT_SYMBOL_GPL(put_online_cpus_atomic);
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