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

[Waiman Long <longman@redhat.com>]

On 9/21/22 18:17, Steven Rostedt wrote:
> On Wed, 21 Sep 2022 09:21:52 -0400
> Waiman Long <longman@redhat.com> 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@redhat.com>
>> ---
>>   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());

Sorry, I missed that.


>
>>   
>>   	/* 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.
Right.
>
>>   }
>>   
>>   /**
>> @@ -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()).
OK.
>
>>   
>>   	__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

Thanks for the comments, will modify the patch as suggested.

Cheers,
Longman