On Friday May 9, snitzer@xxxxxxxxx wrote:
> On Thu, May 8, 2008 at 9:40 PM, Neil Brown <neilb@xxxxxxx> wrote:
> >
> > On Thursday May 8, snitzer@xxxxxxxxx wrote:
> > > On Thu, May 8, 2008 at 2:13 AM, Neil Brown <neilb@xxxxxxx> wrote:
> > > > On Tuesday May 6, snitzer@xxxxxxxxx wrote:
> > > > >
> > > > > It looks like bitmap_update_sb()'s incrementing of events_cleared (on
> > > > > behalf of the local member) could be racing with the fact that the NBD
> > > > > member becomes faulty (whereby making the array degraded). This
> > > > > allows the events_cleared to reflect a clean->dirty transition last
> > > > > occurred before the array became degraded. My reasoning is: If it was
> > > > > a clean->dirty transition the bitmap still has the associated dirty
> > > > > bit set in the local member's bitmap, so using the bitmap to resync is
> > > > > valid.
> > > > >
> > > > > thanks,
> > > > > Mike
> > > >
> > > > Thanks for persisting. I think I understand what is going on now.
> > > >
> > > > How about this patch? It is similar to your, but instead of depending
> > > > on the odd/even state of the event counter, it directly checks the
> > > > clean/dirty state of the array.
> > >
> > > Hi Neil,
> > >
> > > Your revised patch works great and is obviously cleaner.
> >
> > But I'm still not happy with it :-(
> > I suspect there might be other cases where it will still do the wrong
> > thing.
> > The real problem is that we are updating events_cleared to early. We
> > are setting to the new event counter before that is even written out.
> >
> > So I've come up with this patch, which I think more clearly
> > encapsulated what events_cleared means. It is now set to the current
> > 'events' counter immediately before we clear any bit.
> >
> > If you could test it, I'd really appreciate it.
>
> Unfortunately my testing with this patch results in a full resync.
>
> Here is the state of the array after shutdown:
> # mdadm -X /dev/nbd0 /dev/sdq
> Filename : /dev/nbd0
> Magic : 6d746962
> Version : 4
> UUID : 7140cc3c:8681416c:12c5668a:984ca55d
> Events : 896
> Events Cleared : 897
Events Cleared is *larger* than Events!!! Is that repeatable? I can
only see it happening if a very small race were lost. You don't have
any other patches in there do you?
>
> Was I supposed to use this latest patch in combination with your
> previous patch (to validate_super)? Because you'll note that with
> your most recent patch nbd0's events (ev1) is still one less than
> sdq's events_cleared. As such the validate_super's "ev1 <
> mddev->bitmap->events_cleared" check triggers a full rebuild.
No, you weren't suppose to combine it with the previous patch.
This patch should close the race, though I still find it hard to
believe that you lost the race.
NeilBrown
Signed-off-by: Neil Brown <neilb@xxxxxxx>
### Diffstat output
./drivers/md/bitmap.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)
diff .prev/drivers/md/bitmap.c ./drivers/md/bitmap.c
--- .prev/drivers/md/bitmap.c 2008-05-09 11:02:13.000000000 +1000
+++ ./drivers/md/bitmap.c 2008-05-09 16:00:07.000000000 +1000
@@ -465,8 +465,6 @@ void bitmap_update_sb(struct bitmap *bit
spin_unlock_irqrestore(&bitmap->lock, flags);
sb = (bitmap_super_t *)kmap_atomic(bitmap->sb_page, KM_USER0);
sb->events = cpu_to_le64(bitmap->mddev->events);
- if (!bitmap->mddev->degraded)
- sb->events_cleared = cpu_to_le64(bitmap->mddev->events);
kunmap_atomic(sb, KM_USER0);
write_page(bitmap, bitmap->sb_page, 1);
}
@@ -1094,9 +1092,21 @@ void bitmap_daemon_work(struct bitmap *b
} else
spin_unlock_irqrestore(&bitmap->lock, flags);
lastpage = page;
-/*
- printk("bitmap clean at page %lu\n", j);
-*/
+
+ /* We are possibly going to clear some bits, so make
+ * sure that events_cleared is up-to-date.
+ */
+ if (bitmap->events_cleared < bitmap->mddev->events) {
+ bitmap_super_t *sb;
+ bitmap->events_cleared = bitmap->mddev->events;
+ wait_event(mddev->sb_wait,
+ !test_bit(MD_CHANGE_CLEAN, &mddev->flags));
+ sb = kmap_atomic(bitmap->sb_page, KM_USER0);
+ sb->events_cleared =
+ cpu_to_le64(bitmap->events_cleared);
+ kunmap_atomic(sb, KM_USER0);
+ write_page(bitmap, bitmap->sb_page, 1);
+ }
spin_lock_irqsave(&bitmap->lock, flags);
clear_page_attr(bitmap, page, BITMAP_PAGE_CLEAN);
}
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