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

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Date:	Mon, 18 May 2015 21:36:25 +0200
From:	Patrick Marlier <patrick.marlier@...il.com>
To:	NeilBrown <neilb@...e.de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	"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 <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, tglx@...utronix.de,
	peterz@...radead.org, dhowells@...hat.com,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	oleg@...hat.com, pranith kumar <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 05/18/2015 03:53 PM, Patrick Marlier wrote:
> On Mon, May 18, 2015 at 4:06 AM, NeilBrown <neilb@...e.de> wrote:
>> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
>> <patrick.marlier@...il.com> wrote:
>>> On 05/13/2015 04:58 AM, NeilBrown wrote:
>>>> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>> On Tue, 12 May 2015 15:46:26 -0700
>>>>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
>>>>> What comes after this is:
>>>>>
>>>>>     list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
>>>>>             if (rdev->raid_disk >= 0 &&
>>>>>
>>>>> Now the original code had:
>>>>>
>>>>>     rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
>>>>>
>>>>> Where &mddev->disks would return the address of the disks field of
>>>>> mddev which is a list head. Then it would get the 'same_set' offset,
>>>>> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
>>>>> isn't used, as the list_for_each_entry_continue_rcu() has:
>>>>>
>>>>> #define list_for_each_entry_continue_rcu(pos, head, member)                \
>>>>>     for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
>>>>>          &pos->member != (head);    \
>>>>>          pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
>>>>>
>>>>> Thus the first use of pos is pos->member.next or:
>>>>>
>>>>>     mddev->disks.next
>>>>>
>>>>> But now you converted it to rdev = mddev->disks.next, which means the
>>>>> first use is:
>>>>>
>>>>>     pos = mddev->disks.next->next
>>>>>
>>>>> I think you are skipping the first element here.
>>>
>>>
>>> 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.
>
> Here I meant "list_entry_rcu" (in include/linux/rculist.h) not the
> change to drivers/md/bitmap.c.
>
>
>>> 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?
>
> Do not apply the patch on drivers/md/bitmap.c but only on
> include/linux/rculist.h and you will see that the compilation fails.

Here an example of the compilation error in drivers/md/bitmap.c

In file included from include/linux/sched.h:17:0,
                  from include/linux/blkdev.h:4,
                  from drivers/md/bitmap.c:18:
drivers/md/bitmap.c: In function ‘next_active_rdev’:
include/linux/compiler.h:469:24: error: lvalue required as unary ‘&’ operand
   (volatile typeof(x) *)&(x); })
                         ^
include/linux/kernel.h:800:49: note: in definition of macro ‘container_of’
   const typeof( ((type *)0)->member ) *__mptr = (ptr); \
                                                  ^
include/linux/compiler.h:470:26: note: in expansion of macro ‘__ACCESS_ONCE’
  #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
                           ^
include/linux/rcupdate.h:630:26: note: in expansion of macro ‘ACCESS_ONCE’
   typeof(p) _________p1 = ACCESS_ONCE(p); \
                           ^
include/linux/rcupdate.h:587:48: note: in expansion of macro 
‘lockless_dereference’
   typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
                                                 ^
include/linux/rcupdate.h:723:2: note: in expansion of macro 
‘__rcu_dereference_check’
   __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
   ^
include/linux/rcupdate.h:746:32: note: in expansion of macro 
‘rcu_dereference_check’
  #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ 
needed? @@@*/
                                 ^
include/linux/rculist.h:251:28: note: in expansion of macro 
‘rcu_dereference_raw’
   container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member); \
                             ^
drivers/md/bitmap.c:184:10: note: in expansion of macro ‘list_entry_rcu’
    rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
           ^

Thanks.
--
Pat
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