Re: [PATCH] mm: fix lockdep build in rcu-protected get_mm_exe_file()

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Tue, Mar 24, 2015 at 9:10 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 03/24, Konstantin Khlebnikov wrote:
>>
>> On 23.03.2015 22:10, Oleg Nesterov wrote:
>>> On 03/23, Davidlohr Bueso wrote:
>>>>
>>>>   void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>>>>   {
>>>>     struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
>>>> -                   !atomic_read(&mm->mm_users) || current->in_execve ||
>>>> -                   lock_is_held(&mm->mmap_sem));
>>>> +                   !atomic_read(&mm->mm_users) || current->in_execve);
>>>
>>> Thanks, looks correct at first glance...
>>>
>>> But can't we remove the ->in_execve check above? and check
>>>
>>>                      atomic_read(&mm->mm_users) <= 1
>>>
>>> instead. OK, this is subjective, I won't insist. Just current->in_execve
>>> looks a bit confusing, it means "I swear, the caller is flush_old_exec()
>>> and this mm is actualy bprm->mm".
>>>
>>> "atomic_read(&mm->mm_users) <= 1" looks a bit more "safe". But again,
>>> I won't insist.
>>
>> Not so safe: this will race with get_task_mm().
>
> How?

I mean rcu/lockdep debug migh race with get_task_mm() and generate
false-positive warning about non-protected rcu_dereference.

>
> If set_mm_exe_file() can race with get_task_mm() then we have a bug.
> And it will be reported ;)
>
>> A lot of proc files grab temporary reference to task mm.
>> But this just a debug -- we can place here "true".
>
> Yeees, probably rcu_dereference_raw() would be even better. set_mm_exe_file()
> must be called only if nobody but us can access this mm.

Yep.

>
> Oleg.
>
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