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From: Mike Snitzer <snitzer@redhat.com>
Date: Sun, 23 Nov 2014 11:53:59 -0500
Message-ID: <CAMM=eLe5mXBfg_W=bjkFN3_pVAYi1WqRr4nOb90aveA3LvaGDg@mail.gmail.com>
Subject: Re: [PATCH ] drivers/md: use proper rcu accessor
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Pranith Kumar <bobby.prani@gmail.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Dipankar Sarma <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>, dvhart@linux.intel.com,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        device-mapper development <dm-devel@redhat.com>
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On Sun, Nov 23, 2014 at 11:40 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.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@shutemov.name>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Fixes: 33423974bfc1 ("dm: Use rcu_dereference() for accessing rcu pointer")
> Cc: Pranith Kumar <bobby.prani@gmail.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@redhat.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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