Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On Mon, May 18, 2015 at 4:06 AM, NeilBrown <neilb@xxxxxxx> wrote:
> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
> <patrick.marlier@xxxxxxxxx> wrote:
>> On 05/13/2015 04:58 AM, NeilBrown wrote:
>> > On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> >> On Tue, 12 May 2015 15:46:26 -0700
>> >> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> 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.

--
Pat
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