Date: Mon, 18 May 2015 09:43:21 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: NeilBrown <neilb@suse.de>
Cc: Patrick Marlier <patrick.marlier@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com, wangyun@linux.vnet.ibm.com
Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage
Message-ID: <20150518094321.2012a66a@gandalf.local.home>
In-Reply-To: <20150518120647.0c3cecd8@notabene.brown>
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	<20150512223853.55e0ed90@grimm.local.home>
	<20150513125839.371ef677@notabene.brown>
	<5557819E.1060001@gmail.com>
	<20150518120647.0c3cecd8@notabene.brown>
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On Mon, 18 May 2015 12:06:47 +1000
NeilBrown <neilb@suse.de> wrote:


> > struct mddev {
> > ...
> > 	struct list_head                disks;
> > ...}
> > 
> > struct list_head {
> >          struct list_head *next, *prev;
> > };
> > 
> > The tricky thing is that "list_entry_rcu" before and after the patch is 
> > reading the same thing.
> 
> No it isn't.
> Before the patch it is passed the address of the 'next' field.  After the
> patch it is passed the contents of the 'next' field.

Right.

> 
> 
> > 
> > However in your case, the change I proposed is probably wrong I trust 
> > you on this side. :) What's your proposal to fix it with the rculist patch?
> 
> What needs fixing?  I don't see anything broken.
> 
> Maybe there is something in this "rculist patch" that I'm missing.  Can you
> point me at it?
> 

Probably some debugging tool like sparse notices that the assignment
isn't a true list entry and complains about it. In other words, I think
the real fix is to fix the debugging tool to ignore this, because the
code is correct, and this is a false positive failure, and is causing
more harm than good, because people are sending out broken patches due
to it.

-- Steve
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