linux-kernel - Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry

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Message-ID: <20150518094321.2012a66a@gandalf.local.home>
Date:	Mon, 18 May 2015 09:43:21 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Patrick Marlier <patrick.marlier@...il.com>,
	"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,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	wangyun@...ux.vnet.ibm.com
Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

On Mon, 18 May 2015 12:06:47 +1000
NeilBrown <neilb@...e.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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