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

[Patrick Marlier]

On 05/18/2015 03:53 PM, Patrick Marlier wrote:
> 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.

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 ânext_active_rdevâ:
include/linux/compiler.h:469:24: error: lvalue required as unary â&â operand
  (volatile typeof(x) *)&(x); })
                        ^
include/linux/kernel.h:800:49: note: in definition of macro âcontainer_ofâ
  const typeof( ((type *)0)->member ) *__mptr = (ptr); \
                                                 ^
include/linux/compiler.h:470:26: note: in expansion of macro â__ACCESS_ONCEâ
 #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
                          ^
include/linux/rcupdate.h:630:26: note: in expansion of macro âACCESS_ONCEâ
  typeof(p) _________p1 = ACCESS_ONCE(p); \
                          ^
include/linux/rcupdate.h:587:48: note: in expansion of macro 
âlockless_dereferenceâ
  typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
                                                ^
include/linux/rcupdate.h:723:2: note: in expansion of macro 
â__rcu_dereference_checkâ
  __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
  ^
include/linux/rcupdate.h:746:32: note: in expansion of macro 
ârcu_dereference_checkâ
 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ 
needed? @@@*/
                                ^
include/linux/rculist.h:251:28: note: in expansion of macro 
ârcu_dereference_rawâ
  container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member); \
                            ^
drivers/md/bitmap.c:184:10: note: in expansion of macro âlist_entry_rcuâ
   rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
          ^

Thanks.
--
Pat
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/