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Subject[PATCH 10/12] task_isolation: ringbuffer: don't interrupt CPUs running isolated tasks on buffer resize
Date
From: Yuri Norov <ynorov@marvell.com>

CPUs running isolated tasks are in userspace, so they don't have to
perform ring buffer updates immediately. If ring_buffer_resize()
schedules the update on those CPUs, isolation is broken. To prevent
that, updates for CPUs running isolated tasks are performed locally,
like for offline CPUs.

A race condition between this update and isolation breaking is avoided
at the cost of disabling per_cpu buffer writing for the time of update
when it coincides with isolation breaking.

Signed-off-by: Alex Belits <abelits@marvell.com>
---
kernel/trace/ring_buffer.c | 61 ++++++++++++++++++++++++++++++++++----
1 file changed, 55 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 61f0e92ace99..6c7479b6411d 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -21,6 +21,7 @@
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/init.h>
+#include <linux/isolation.h>
#include <linux/hash.h>
#include <linux/list.h>
#include <linux/cpu.h>
@@ -1701,6 +1702,37 @@ static void update_pages_handler(struct work_struct *work)
complete(&cpu_buffer->update_done);
}

+static bool update_if_isolated(struct ring_buffer_per_cpu *cpu_buffer,
+ int cpu)
+{
+ bool rv = false;
+
+ if (task_isolation_on_cpu(cpu)) {
+ /*
+ * CPU is running isolated task. Since it may lose
+ * isolation and re-enter kernel simultaneously with
+ * this update, disable recording until it's done.
+ */
+ atomic_inc(&cpu_buffer->record_disabled);
+ /* Make sure, update is done, and isolation state is current */
+ smp_mb();
+ if (task_isolation_on_cpu(cpu)) {
+ /*
+ * If CPU is still running isolated task, we
+ * can be sure that breaking isolation will
+ * happen while recording is disabled, and CPU
+ * will not touch this buffer until the update
+ * is done.
+ */
+ rb_update_pages(cpu_buffer);
+ cpu_buffer->nr_pages_to_update = 0;
+ rv = true;
+ }
+ atomic_dec(&cpu_buffer->record_disabled);
+ }
+ return rv;
+}
+
/**
* ring_buffer_resize - resize the ring buffer
* @buffer: the buffer to resize.
@@ -1784,13 +1816,22 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
if (!cpu_buffer->nr_pages_to_update)
continue;

- /* Can't run something on an offline CPU. */
+ /*
+ * Can't run something on an offline CPU.
+ *
+ * CPUs running isolated tasks don't have to
+ * update ring buffers until they exit
+ * isolation because they are in
+ * userspace. Use the procedure that prevents
+ * race condition with isolation breaking.
+ */
if (!cpu_online(cpu)) {
rb_update_pages(cpu_buffer);
cpu_buffer->nr_pages_to_update = 0;
} else {
- schedule_work_on(cpu,
- &cpu_buffer->update_pages_work);
+ if (!update_if_isolated(cpu_buffer, cpu))
+ schedule_work_on(cpu,
+ &cpu_buffer->update_pages_work);
}
}

@@ -1829,13 +1870,22 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,

get_online_cpus();

- /* Can't run something on an offline CPU. */
+ /*
+ * Can't run something on an offline CPU.
+ *
+ * CPUs running isolated tasks don't have to update
+ * ring buffers until they exit isolation because they
+ * are in userspace. Use the procedure that prevents
+ * race condition with isolation breaking.
+ */
if (!cpu_online(cpu_id))
rb_update_pages(cpu_buffer);
else {
- schedule_work_on(cpu_id,
+ if (!update_if_isolated(cpu_buffer, cpu_id))
+ schedule_work_on(cpu_id,
&cpu_buffer->update_pages_work);
- wait_for_completion(&cpu_buffer->update_done);
+ wait_for_completion(&cpu_buffer->update_done);
+ }
}

cpu_buffer->nr_pages_to_update = 0;
--
2.20.1
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