Messages in this thread | | | Date | Mon, 18 May 2015 21:36:25 +0200 | From | Patrick Marlier <> | Subject | Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage |
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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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