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

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Message-ID: <20150323191055.GA10212@redhat.com>
Date:	Mon, 23 Mar 2015 20:10:55 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	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 03/23, 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.

Oleg.

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