linux-kernel - Re: [PATCH] mm: fix lockdep build in rcu-protected get_mm_exe

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Message-ID: <20150324181016.GA9678@redhat.com>
Date:	Tue, 24 Mar 2015 19:10:16 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	Davidlohr Bueso <dave@...olabs.net>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lockdep build in rcu-protected
	get_mm_exe_file()

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?

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.

Oleg.

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