Re: [PATCH 2/2] tracing: Use proper do_arch_spin_lock() API

[Steven Rostedt]

On Wed, 21 Sep 2022 09:21:52 -0400
Waiman Long <longman@xxxxxxxxxx> wrote:

> It was found that some tracing functions acquire a arch_spinlock_t with
> preemption and irqs enabled.  That can be problematic in case preemption
> happens after acquiring the lock. Use the proper do_arch_spin_lock()
> API with preemption disabled to make sure that this won't happen unless
> it is obvious that either preemption or irqs has been disabled .
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/trace/trace.c | 52 ++++++++++++++++++++------------------------
>  1 file changed, 24 insertions(+), 28 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d3005279165d..cbb8520842ad 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1193,12 +1193,12 @@ void *tracing_cond_snapshot_data(struct trace_array *tr)
>  {
>  	void *cond_data = NULL;
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

This should actually disable interrupts and not preemption.

>  
>  	if (tr->cond_snapshot)
>  		cond_data = tr->cond_snapshot->cond_data;
>  
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);
>  
>  	return cond_data;
>  }
> @@ -1334,9 +1334,9 @@ int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data,
>  		goto fail_unlock;
>  	}
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

Same here.

>  	tr->cond_snapshot = cond_snapshot;
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);
>  
>  	mutex_unlock(&trace_types_lock);
>  
> @@ -1363,7 +1363,7 @@ int tracing_snapshot_cond_disable(struct trace_array *tr)
>  {
>  	int ret = 0;
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

And here.

>  
>  	if (!tr->cond_snapshot)
>  		ret = -EINVAL;
> @@ -1372,7 +1372,7 @@ int tracing_snapshot_cond_disable(struct trace_array *tr)
>  		tr->cond_snapshot = NULL;
>  	}
>  
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);
>  
>  	return ret;
>  }
> @@ -1819,7 +1819,7 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
>  		return;
>  	}
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

Nothing here is needed, as interrupts had better be disabled when this
function is called. And there's already a:

	WARN_ON_ONCE(!irqs_disabled());

>  
>  	/* Inherit the recordable setting from array_buffer */
>  	if (ring_buffer_record_is_set_on(tr->array_buffer.buffer))
> @@ -1836,7 +1836,7 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
>  	__update_max_tr(tr, tsk, cpu);
>  
>   out_unlock:
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);

Nothing needs to be done here.

>  }
>  
>  /**
> @@ -1862,7 +1862,7 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
>  		return;
>  	}
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

Same here. Interrupts had better be disabled in this function.

>  
>  	ret = ring_buffer_swap_cpu(tr->max_buffer.buffer, tr->array_buffer.buffer, cpu);
>  
> @@ -1880,7 +1880,7 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
>  	WARN_ON_ONCE(ret && ret != -EAGAIN && ret != -EBUSY);
>  
>  	__update_max_tr(tr, tsk, cpu);
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);

Nothing to do here.

>  }
>  #endif /* CONFIG_TRACER_MAX_TRACE */
>  
> @@ -2413,7 +2413,7 @@ static int trace_save_cmdline(struct task_struct *tsk)
>  	 * nor do we want to disable interrupts,
>  	 * so if we miss here, then better luck next time.
>  	 */
> -	if (!arch_spin_trylock(&trace_cmdline_lock))
> +	if (!do_arch_spin_trylock(&trace_cmdline_lock))

This is called within the scheduler and wake up, so interrupts had better
be disabled (run queue lock is held).

>  		return 0;
>  
>  	idx = savedcmd->map_pid_to_cmdline[tpid];
> @@ -2427,7 +2427,7 @@ static int trace_save_cmdline(struct task_struct *tsk)
>  	savedcmd->map_cmdline_to_pid[idx] = tsk->pid;
>  	set_cmdline(idx, tsk->comm);
>  
> -	arch_spin_unlock(&trace_cmdline_lock);
> +	do_arch_spin_unlock(&trace_cmdline_lock);

Nothing to do here.

>  
>  	return 1;
>  }
> @@ -2461,13 +2461,11 @@ static void __trace_find_cmdline(int pid, char comm[])
>  
>  void trace_find_cmdline(int pid, char comm[])
>  {
> -	preempt_disable();
> -	arch_spin_lock(&trace_cmdline_lock);
> +	do_arch_spin_lock(&trace_cmdline_lock);

Keep this as is (with the open coded preempt_disable()).

>  
>  	__trace_find_cmdline(pid, comm);
>  
> -	arch_spin_unlock(&trace_cmdline_lock);
> -	preempt_enable();
> +	do_arch_spin_unlock(&trace_cmdline_lock);
>  }
>  
>  static int *trace_find_tgid_ptr(int pid)
> @@ -5829,8 +5827,7 @@ static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
>  	void *v;
>  	loff_t l = 0;
>  
> -	preempt_disable();
> -	arch_spin_lock(&trace_cmdline_lock);
> +	do_arch_spin_lock(&trace_cmdline_lock);

This too.

>  
>  	v = &savedcmd->map_cmdline_to_pid[0];
>  	while (l <= *pos) {
> @@ -5844,8 +5841,7 @@ static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
>  
>  static void saved_cmdlines_stop(struct seq_file *m, void *v)
>  {
> -	arch_spin_unlock(&trace_cmdline_lock);
> -	preempt_enable();
> +	do_arch_spin_unlock(&trace_cmdline_lock);

And this.

>  }
>  
>  static int saved_cmdlines_show(struct seq_file *m, void *v)
> @@ -5890,9 +5886,9 @@ tracing_saved_cmdlines_size_read(struct file *filp, char __user *ubuf,
>  	char buf[64];
>  	int r;
>  
> -	arch_spin_lock(&trace_cmdline_lock);
> +	do_arch_spin_lock(&trace_cmdline_lock);

Yeah, we should add preempt_disable() here.

>  	r = scnprintf(buf, sizeof(buf), "%u\n", savedcmd->cmdline_num);
> -	arch_spin_unlock(&trace_cmdline_lock);
> +	do_arch_spin_unlock(&trace_cmdline_lock);
>  
>  	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
>  }
> @@ -5917,10 +5913,10 @@ static int tracing_resize_saved_cmdlines(unsigned int val)
>  		return -ENOMEM;
>  	}
>  
> -	arch_spin_lock(&trace_cmdline_lock);
> +	do_arch_spin_lock(&trace_cmdline_lock);

And here.

>  	savedcmd_temp = savedcmd;
>  	savedcmd = s;
> -	arch_spin_unlock(&trace_cmdline_lock);
> +	do_arch_spin_unlock(&trace_cmdline_lock);
>  	free_saved_cmdlines_buffer(savedcmd_temp);
>  
>  	return 0;
> @@ -6373,10 +6369,10 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
>  
>  #ifdef CONFIG_TRACER_SNAPSHOT
>  	if (t->use_max_tr) {
> -		arch_spin_lock(&tr->max_lock);
> +		do_arch_spin_lock(&tr->max_lock);

Add preemption disabling.

>  		if (tr->cond_snapshot)
>  			ret = -EBUSY;
> -		arch_spin_unlock(&tr->max_lock);
> +		do_arch_spin_unlock(&tr->max_lock);
>  		if (ret)
>  			goto out;
>  	}
> @@ -7436,10 +7432,10 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
>  		goto out;
>  	}
>  
> -	arch_spin_lock(&tr->max_lock);
> +	do_arch_spin_lock(&tr->max_lock);

And this should disable interrupts first.

-- Steve

>  	if (tr->cond_snapshot)
>  		ret = -EBUSY;
> -	arch_spin_unlock(&tr->max_lock);
> +	do_arch_spin_unlock(&tr->max_lock);
>  	if (ret)
>  		goto out;
>