Messages in this thread | | | Date | Sat, 16 May 2015 19:42:54 +0200 | From | Patrick Marlier <> | Subject | Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage |
| |
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: >> >>> From: Patrick Marlier <patrick.marlier@gmail.com> >>> >>> Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com> >>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> >>> --- >>> drivers/md/bitmap.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c >>> index 2bc56e2a3526..32901772e4ee 100644 >>> --- a/drivers/md/bitmap.c >>> +++ b/drivers/md/bitmap.c >>> @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde >>> rcu_read_lock(); >>> if (rdev == NULL) >>> /* start at the beginning */ >>> - rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); >>> + rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set); >> >> Hmm, this changes the semantics. >> >> The original code looks nasty, I first thought it was broken, but it >> seems to work out of sheer luck (or clever hack) > > Definitely a clever hack - no question of "luck" here :-) > > It might makes sense to change it to use list_for_each_entry_from_rcu() > > if (rdev == NULL) > rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set); > else { > rdev_dec_pending(rdev, mddev); > rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set); > } > list_for_each_entry_from_rcu(rdev, ....) > > but there isn't a "list_next_entry_rcu".... > > > Also, it would have been polity to at least 'cc' them Maintainer of this code > in the original patch - no?
Sure my bad. I hesitated to CC maintainers. I was almost sure that it will be rejected so I wanted to avoid noise.
> > Thanks, > NeilBrown > >> >>> else { >>> /* release the previous rdev and start from there. */ >>> rdev_dec_pending(rdev, mddev); >> >> >> 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.
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?
PS: In the rculist patch I proposed, I avoid the store and the atomic reload in the stack variable __ptr. (yeap, the rcu_dereference_raw/ACCESS_ONCE is a bit confusing because it implicitly do & on the parameter).
Thanks. -- Pat
| |