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

[Waiman Long]

On 9/21/22 18:17, Steven Rostedt wrote:
> 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());

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