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

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Date:	Sun, 13 Sep 2015 12:06:33 +0200
From:	Patrick Marlier <patrick.marlier@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Steven Rostedt <rostedt@...dmis.org>, NeilBrown <neilb@...e.de>,
	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 09/12/2015 01:05 AM, Paul E. McKenney wrote:
> On Tue, May 19, 2015 at 03:07:25PM -0700, Paul E. McKenney wrote:
>> On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
>>> 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.
>>
>> OK, finally did the history trawling that I should have done to begin with.
>>
>> Back in 2010, Arnd added the __rcu pointer checking in sparse.
>> But the RCU list primitives were used on non-RCU-protected lists, so
>> some casting pain was required to avoid sparse complaints.  (Keep in
>> mind that the list_head structure does not mark ->next with __rcu.)
>> Arnd's workaround was to copy the pointer to the local stack, casting
>> it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
>> traversal of an RCU-protected pointer.
>>
>> This of course resulted in an extraneous load from the stack, which
>> Patrick noticed in his performance work, and which motivated him to send
>> the patches.
>>
>> Perhaps what I should do is create an rcu_dereference_nocheck() for use
>> in list traversals, that omits the sparse checking.  That should get rid
>> of both the sparse warnings and the strange casts.
>>
>> The code in md probably needs to change in any case, as otherwise we are
>> invoking rcu_dereference_whatever() on a full struct list_head rather
>> than on a single pointer.  Or am I missing something here?
>
> Finally getting back to this one...
>
> I switched to lockless_dereference() instead of rcu_dereference_raw(),
> and am running it through the testing gamut.  Patrick, are you OK with
> this change?

Paul,

This sounds good to me. It should fix the performance issue (will check 
with my benchmark).
I think for drivers/md/bitmap.c:next_active_rdev() the problem was fixed 
but do you know if it also fixed for net/netfilter/core.c:nf_hook_slow()?

Thanks.
--
Pat
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