Commit 4f48f8b7 authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Steven Rostedt

tracing: Fix circular dead lock in stack trace

When we cat <debugfs>/tracing/stack_trace, we may cause circular lock:
sys_read()
  t_start()
     arch_spin_lock(&max_stack_lock);

  t_show()
     seq_printf(), vsnprintf() .... /* they are all trace-able,
       when they are traced, max_stack_lock may be required again. */

The following script can trigger this circular dead lock very easy:
#!/bin/bash

echo 1 > /proc/sys/kernel/stack_tracer_enabled

mount -t debugfs xxx /mnt > /dev/null 2>&1

(
# make check_stack() zealous to require max_stack_lock
for ((; ;))
{
	echo 1 > /mnt/tracing/stack_max_size
}
) &

for ((; ;))
{
	cat /mnt/tracing/stack_trace > /dev/null
}

To fix this bug, we increase the percpu trace_active before
require the lock.
Reported-by: default avatarLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <4B67D4F9.9080905@cn.fujitsu.com>
Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent ab658321
......@@ -157,6 +157,7 @@ stack_max_size_write(struct file *filp, const char __user *ubuf,
unsigned long val, flags;
char buf[64];
int ret;
int cpu;
if (count >= sizeof(buf))
return -EINVAL;
......@@ -171,9 +172,20 @@ stack_max_size_write(struct file *filp, const char __user *ubuf,
return ret;
local_irq_save(flags);
/*
* In case we trace inside arch_spin_lock() or after (NMI),
* we will cause circular lock, so we also need to increase
* the percpu trace_active here.
*/
cpu = smp_processor_id();
per_cpu(trace_active, cpu)++;
arch_spin_lock(&max_stack_lock);
*ptr = val;
arch_spin_unlock(&max_stack_lock);
per_cpu(trace_active, cpu)--;
local_irq_restore(flags);
return count;
......@@ -206,7 +218,13 @@ t_next(struct seq_file *m, void *v, loff_t *pos)
static void *t_start(struct seq_file *m, loff_t *pos)
{
int cpu;
local_irq_disable();
cpu = smp_processor_id();
per_cpu(trace_active, cpu)++;
arch_spin_lock(&max_stack_lock);
if (*pos == 0)
......@@ -217,7 +235,13 @@ static void *t_start(struct seq_file *m, loff_t *pos)
static void t_stop(struct seq_file *m, void *p)
{
int cpu;
arch_spin_unlock(&max_stack_lock);
cpu = smp_processor_id();
per_cpu(trace_active, cpu)--;
local_irq_enable();
}
......
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