linux-kernel - Re: [PATCH ] drivers/md: use proper rcu accessor

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Message-ID: <CAMM=eLe5mXBfg_W=bjkFN3_pVAYi1WqRr4nOb90aveA3LvaGDg@mail.gmail.com>
Date:	Sun, 23 Nov 2014 11:53:59 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Pranith Kumar <bobby.prani@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	"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>,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: [PATCH ] drivers/md: use proper rcu accessor

On Sun, Nov 23, 2014 at 11:40 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> rcu_dereference() should be used in sections protected by rcu_read_lock.
>
> For writers, holding some kind of mutex or lock,
> rcu_dereference_protected() is the way to go, adding explicit lockdep
> bits.
>
> In __unbind(), although there is no mutex or lock held, we are about
> to free the mapped device, so can use the constant '1' instead of a
> lockdep_is_held()

That isn't true.  dm_hash_remove_all() -- which calls dm_destroy --
holds _hash_lock.  Why leave __unbind() brittle in the face of future
DM locking changes?

> Reported-by: Kirill A. Shutemov <kirill@...temov.name>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: 33423974bfc1 ("dm: Use rcu_dereference() for accessing rcu pointer")
> Cc: Pranith Kumar <bobby.prani@...il.com>

Hi Eric,

I'll pick this up once I get clarification for why your __unbind
change is safe.. but it really would've helped if you cc'd
dm-devel@...hat.com or myself directly (not a single person that you
cc'd actively maintains DM).

Hopefully these DM rcu "fixes" are finished after this.

Thanks,
Mike
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