[PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

[PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

[zhangwei(Jovi)]

It's reasonable to call __ftrace_dump function not only once,
so remove the dump_ran variable checking.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
---
 kernel/trace/trace.c |    6 ------
 1 file changed, 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 090eddb..4cec7b8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5106,17 +5106,12 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
 	/* use static because iter can be a bit big for the stack */
 	static struct trace_iterator iter;
 	unsigned int old_userobj;
-	static int dump_ran;
 	unsigned long flags;
 	int cnt = 0, cpu;

 	/* only one dump */
 	local_irq_save(flags);
 	arch_spin_lock(&ftrace_dump_lock);
-	if (dump_ran)
-		goto out;
-
-	dump_ran = 1;

 	tracing_off();

@@ -5206,7 +5201,6 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
 		tracing_on();
 	}

- out:
 	arch_spin_unlock(&ftrace_dump_lock);
 	local_irq_restore(flags);
 }
-- 
1.7.9.7

Re: [PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

[Steven Rostedt]

On Mon, 2013-03-11 at 15:13 +0800, zhangwei(Jovi) wrote:
> It's reasonable to call __ftrace_dump function not only once,
> so remove the dump_ran variable checking.

This needs a little more work. On an oops, I only want it dumped once,
because a crash can cause another crash while its dumping, and without
that check in will corrupt the buffer.

Now, we have things like ctrl^z that also does a dump where we don't
want to disable it. Cleaning this up has been on my todo list for a
while. I may go ahead and clean that up myself.

-- Steve

> 
> Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
> ---
>  kernel/trace/trace.c |    6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 090eddb..4cec7b8 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -5106,17 +5106,12 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
>  	/* use static because iter can be a bit big for the stack */
>  	static struct trace_iterator iter;
>  	unsigned int old_userobj;
> -	static int dump_ran;
>  	unsigned long flags;
>  	int cnt = 0, cpu;
> 
>  	/* only one dump */
>  	local_irq_save(flags);
>  	arch_spin_lock(&ftrace_dump_lock);
> -	if (dump_ran)
> -		goto out;
> -
> -	dump_ran = 1;
> 
>  	tracing_off();
> 
> @@ -5206,7 +5201,6 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
>  		tracing_on();
>  	}
> 
> - out:
>  	arch_spin_unlock(&ftrace_dump_lock);
>  	local_irq_restore(flags);
>  }

Re: [PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

[Jovi Zhang]

On Mon, Mar 11, 2013 at 10:08 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Mon, 2013-03-11 at 15:13 +0800, zhangwei(Jovi) wrote:
>> It's reasonable to call __ftrace_dump function not only once,
>> so remove the dump_ran variable checking.
>
> This needs a little more work. On an oops, I only want it dumped once,
> because a crash can cause another crash while its dumping, and without
> that check in will corrupt the buffer.
For reclusive dumping, it's already under the protection of
ftrace_dump_lock spinlock,
I missed something? would you explain more on this case?
>
> Now, we have things like ctrl^z that also does a dump where we don't
> want to disable it. Cleaning this up has been on my todo list for a
> while. I may go ahead and clean that up myself.
Nice, please ignore this patch if that code will be cleanup.
Thanks.

>
> -- Steve
>
>>
>> Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
>> ---
>>  kernel/trace/trace.c |    6 ------
>>  1 file changed, 6 deletions(-)
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index 090eddb..4cec7b8 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -5106,17 +5106,12 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
>>       /* use static because iter can be a bit big for the stack */
>>       static struct trace_iterator iter;
>>       unsigned int old_userobj;
>> -     static int dump_ran;
>>       unsigned long flags;
>>       int cnt = 0, cpu;
>>
>>       /* only one dump */
>>       local_irq_save(flags);
>>       arch_spin_lock(&ftrace_dump_lock);
>> -     if (dump_ran)
>> -             goto out;
>> -
>> -     dump_ran = 1;
>>
>>       tracing_off();
>>
>> @@ -5206,7 +5201,6 @@ __ftrace_dump(bool disable_tracing, enum ftrace_dump_mode oops_dump_mode)
>>               tracing_on();
>>       }
>>
>> - out:
>>       arch_spin_unlock(&ftrace_dump_lock);
>>       local_irq_restore(flags);
>>  }
>
>
> --
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Re: [PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

[Steven Rostedt]

On Mon, 2013-03-11 at 23:35 +0800, Jovi Zhang wrote:
> On Mon, Mar 11, 2013 at 10:08 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Mon, 2013-03-11 at 15:13 +0800, zhangwei(Jovi) wrote:
> >> It's reasonable to call __ftrace_dump function not only once,
> >> so remove the dump_ran variable checking.
> >
> > This needs a little more work. On an oops, I only want it dumped once,
> > because a crash can cause another crash while its dumping, and without
> > that check in will corrupt the buffer.
> For reclusive dumping, it's already under the protection of
> ftrace_dump_lock spinlock,
> I missed something? would you explain more on this case?

Actually, it matters if it was called from NMI context or not. If an NMI
triggered and did a dump while a reader was reading the buffer, the NMI
can corrupt the buffer. It will print fine for the NMI, but if the
reader continues, there's a chance it can get messed up and corrupt the
buffer. The "dump once" is a paranoid method to say we only dump it once
on oops and stop (hence the ftrace_kill() there too).

Now I think there's a possible deadlock here as well. If the dump was
caused by something other than an NMI lock up, and while it is dumping
the NMI goes off and triggers a bug, it too can enter this and that
arch_spin_lock() will cause a deadlock. This will be something I need to
clean up as well.

Thanks for calling my attention to this.

-- Steve