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

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Date:	Fri, 21 Nov 2014 16:58:17 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, 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 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.

-- 
 Kirill A. Shutemov
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