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

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Date:	Tue, 24 Mar 2015 20:02:29 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Davidlohr Bueso <dave@...olabs.net>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <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 Tue, Mar 24, 2015 at 9:10 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >>>
> >>> "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.

Still can't understand, I think it can't... and if it could, then this
warning would not be false positive.

Anut this doesn't matter because we seem to agree this check should go away.

> > 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.

Great. Davidlohr will you agree?

Oleg.

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