linux-kernel - Re: [PATCH] mm: rcu-protected get_mm_exe

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Message-ID: <20150316163521.GA9306@redhat.com>
Date:	Mon, 16 Mar 2015 17:35:21 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] mm: rcu-protected get_mm_exe_file()

On 03/16, Konstantin Khlebnikov wrote:
>
> On 16.03.2015 17:07, Oleg Nesterov wrote:
>> I mean, I think we can do another cleanup on top of this change.
>>
>> 	1. set_mm_exe_file() should be called by exit/exec only, so
>> 	   it should use
>>
>> 		rcu_dereference_protected(mm->exe_file,
>> 					atomic_read(&mm->mm_users) <= 1);
>>
>> 	2. prctl() should not use it, it can do
>>
>> 	   get_file(new_exe);
>> 	   old_exe = xchg(&mm->exe_file);
>> 	   if (old_exe)
>> 	   	fput(old_exe);
>
> I think smp_mb() is required before xchg() or
> probably this stuff should be hidden inside yet another magic RCU macro
> ( with two screens of comments =)

Not really, xchg() implies mb's on both sides. As any other atomic operation
which returns the result.

(And in fact we do not even need rcu_assign_pointer() in set_mm_exe_file(),
 get_mm_exe_file() could do READ_ONCE() + inc_not_zero(). But this is off-
 topic, and of course rcu_* helpers look better)

Oleg.

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