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

kernel/trace/trace_stack.c

+24 −0
Original line number Diff line number Diff line
@@ -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();

}