linux-kernel - Re: [PATCH tip/core/rcu 3/9] drivers/md: Use rcu

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Message-ID: <CAJhHMCCE_p+xTibLZwoS5OqW--NO_uEVsW6tfN6LcgZGBquBTQ@mail.gmail.com>
Date:	Sun, 23 Nov 2014 07:21:44 -0500
From:	Pranith Kumar <bobby.prani@...il.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH tip/core/rcu 3/9] drivers/md: Use rcu_dereference() for
 accessing rcu pointer

On Fri, Nov 21, 2014 at 9:58 AM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> On Fri, Nov 21, 2014 at 09:30:36AM -0500, Pranith Kumar wrote:
>> On 11/21/2014 08:31 AM, Kirill A. Shutemov wrote:
>> > On Tue, Oct 28, 2014 at 03:09:56PM -0700, Paul E. McKenney wrote:
>> >> From: Pranith Kumar <bobby.prani@...il.com>
>> >>
>> >> Got Paul's email wrong the first time.
>> >>
>> >> The map field in 'struct mapped_device' is an rcu pointer. Use rcu_dereference()
>> >> while accessing it.
>> >>
>> >> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
>> >> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> >
>> > On current -next I see this:
>> >
>> > [    6.388264] ===============================
>> > [    6.389571] [ INFO: suspicious RCU usage. ]
>> > [    6.390869] 3.18.0-rc5-next-20141121-08303-g44cae4530372 #2 Not tainted
>> > [    6.392185] -------------------------------
>> > [    6.393479] /home/kas/git/public/linux/drivers/md/dm.c:2853 suspicious rcu_dereference_check() usage!
>> > [    6.394801]
>> > other info that might help us debug this:
>> >
>>
>> Hi Kirill,
>>
>> We are dereferencing an RCU pointer with the suspend_lock held which is causing this warning.
>>
>> Can you please check if the following patch helps? Thanks!
>
> Nope. The same issue.
>
> IIUC, the problem is that you dereference pointer outside rcu_read_lock()
> section, not that suspend_lock is held.
>

I am not sure we should be taking rcu_read_lock() there as I am not
sure how long that critical section might last. Can someone who is
more familiar with the code take a look?

I will try to look for a solution too in the mean time.

Thanks!
-- 
Pranith
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