linux-kernel - Re: [PATCH 2/3] btrfs: fix suspicious RCU in BTRFS_IOC_DEV

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Message-ID: <20141201031555.GB14040@mew>
Date:	Sun, 30 Nov 2014 19:15:55 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Pranith Kumar <bobby.prani+lkml@...il.com>
Cc:	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	Joe Perches <joe@...ches.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/3] btrfs: fix suspicious RCU in BTRFS_IOC_DEV_INFO

On Sun, Nov 30, 2014 at 10:11:41AM -0500, Pranith Kumar wrote:
> On Sun, Nov 30, 2014 at 3:26 AM, Omar Sandoval <osandov@...ndov.com> wrote:
> > A naked read of the value of an RCU pointer isn't safe. Put the whole access in
> > an RCU critical section, not just the pointer dereference.
> >
> > Signed-off-by: Omar Sandoval <osandov@...ndov.com>
> 
> You can use rcu_access_pointer() in the if() condition check rather
> than increasing the read critical section. We should try to keep the
> critical section as small as possible.
> 
> Also, since we have rcu_str_deref() we can use that instead of
> rcu_dereference() on device->name. Thoughts?
> 
That's right, I forgot about rcu_access_pointer. The difference is probably
negligible, and I doubt the performance of this ioctl is very important. Since
we're going to be dereferencing the pointer anyways in some (most?) cases, I
think this is a bit more readable.

-- 
Omar
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