linux-kernel - Re: [PATCH 2/3] tracing/stack-tracer: fix locking

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Message-ID: <492938A4.9020502@gmail.com>
Date:	Sun, 23 Nov 2008 13:04:04 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	srostedt@...hat.com, a.p.zijlstra@...llo.nl, sandmann@...mi.au.dk,
	linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk
Subject: Re: [PATCH 2/3] tracing/stack-tracer: fix locking

On 2008-11-23 13:01, Ingo Molnar wrote:
> * Török Edwin <edwintorok@...il.com> wrote:
>
>   
>> On 2008-11-23 12:52, Ingo Molnar wrote:
>>     
>>> * Török Edwin <edwintorok@...il.com> wrote:
>>>
>>>   
>>>       
>>>> +		down_read(&mm->mmap_sem);
>>>> +		vma = find_vma(mm, ip);
>>>>  		if (vma) {
>>>>  			file = vma->vm_file;
>>>>  			vmstart = vma->vm_start;
>>>>  		}
>>>> +		up_read(&mm->mmap_sem);
>>>>  	}
>>>>  	if (file) {
>>>>  		ret = trace_seq_path(s, &file->f_path);
>>>>     
>>>>         
>>> and now it's "file" that is held without a reference and possibly 
>>> racy.
>>>   
>>>       
>> I'll move the (file) inside the mmap_sem too, since file is part of 
>> vma, and you need mmap_sem to access the vma nobody can modify 
>> (file) if they don't hold mmap_sem. Am I right?
>>     
>
> yeah - you can use the mm lock for that as the vma holds a reference 
> to the file. This works as long as none of the seq functions called 
> with this lock held can provoke a user pagefault (they shouldnt, but i 
> havent checked in detail).
>   

It calls d_path, that access only kernel structures AFAICT, so it can't
provoke a *user* pagefault.
Not sure about kernel pagefaults, but they shouldn't matter.

Best regards,
--Edwin
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