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

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Date:	Tue, 24 Mar 2015 20:13:31 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Oleg Nesterov <oleg@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>
CC:	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 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().
A lot of proc files grab temporary reference to task mm.
But this just a debug -- we can place here "true".

>
> Oleg.
>

-- 
Konstantin
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