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    SubjectRe: [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@suse.de> wrote:
    >> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
    >> <patrick.marlier@gmail.com> wrote:
    >>> On 05/13/2015 04:58 AM, NeilBrown wrote:
    >>>> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
    >>>>> On Tue, 12 May 2015 15:46:26 -0700
    >>>>> "Paul E. McKenney" <paulmck@linux.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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