linux-ext4 - Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference

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Date:	Mon, 22 Nov 2010 09:38:49 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	"Ted Ts'o" <tytso@....edu>, Minchan Kim <minchan.kim@...il.com>,
	linux-ext4@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Eric Sandeen <sandeen@...hat.com>
Subject: Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference_check() usage

On Mon, Nov 22, 2010 at 2:37 AM, Ted Ts'o <tytso@....edu> wrote:
> On Mon, Nov 22, 2010 at 12:39:49AM +0900, Minchan Kim wrote:
>>
>> I think it's no problem.
>>
>> That's because migration always holds lock_page on the file page.
>> So the page couldn't remove from radix.
>
> It may be "ok" in that it won't cause a race, but it still leaves an
> unsightly warning if LOCKDEP is enabled, and LOCKDEP warnings will
> cause /proc_lock_stat to be disabled.  So I think it still needs to be
> fixed by adding rcu_read_lock()/rcu_read_unlock() to
> migrate_page_move_mapping().
>
>                                                     - Ted
>

Yes. if it is really "ok" about race, we will add rcu_read_lock with
below comment to prevent false positive.
"suppress RCU lockdep false positives".
But I am not sure it's good although rcu_read_lock is little cost.
Whenever we find false positive, should we add rcu_read_lock to
suppress although it's no problem in real product?
Couldn't we provide following function? (or we might have already it
but I missed it. )

/*
 * Suppress RCU lockdep false positive.
 */
#ifdef CONFIG_PROVE_RCU
#define rcu_read_lock_suppress rcu_read_lock
#else
#define rcu_read_lock_suppress
#endif

-- 
Kind regards,
Minchan Kim
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