linux-kernel - Re: [PATCH 1/4] Add support for userspace stacktraces in tracing/iter

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Date:	Sun, 12 Oct 2008 16:53:06 +0300
From:	Török Edwin <edwintorok@...il.com>
To:	Frédéric Weisbecker <fweisbec@...il.com>
CC:	mingo@...e.hu, srostedt@...hat.com, a.p.zijlstra@...llo.nl,
	sandmann@...mi.au.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] Add support for userspace stacktraces in tracing/iter_ctrl

On 2008-10-12 16:31, Frédéric Weisbecker wrote:
> 2008/10/12 Török Edwin <edwintorok@...il.com>:
>   
>> +static int copy_stack_frame(const void __user *fp, struct stack_frame *frame)
>> +{
>> +       int ret;
>> +
>> +       if (!access_ok(VERIFY_READ, fp, sizeof(*frame)))
>> +               return 0;
>> +
>> +       ret = 1;
>> +       pagefault_disable();
>> +       if (__copy_from_user_inatomic(frame, fp, sizeof(*frame)))
>> +               ret = 0;
>> +       pagefault_enable();
>> +
>> +       return ret;
>> +}
>> +
>>     
>
> Hello Török,
>
> I don't understand why you are using this pagefault disabling and
> atomic copy here.
> Wouldn't a simple copy_from_user actually take of this? You don't seem
> to be in atomic
> context.
>   

Hello,

This function is called from ftrace, as such I could be holding a
spinlock. It can also be called from
__rwsem_do_wake, which takes a spinlock.
This code is from sysprof's copy_stack_frame. Maybe pagefault_disable()
is not needed.

Best regards,
--Edwin
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