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    SubjectRe: inode->i_wb_list corruption.
    On Tue 06-03-12 23:26:08, Wu Fengguang wrote:
    > On Tue, Mar 06, 2012 at 10:03:07PM +0100, Jan Kara wrote:
    > > On Tue 06-03-12 13:51:37, Dave Jones wrote:
    > > > We've had three separate reports against 3.2.x recently where the linked list debugging
    > > > is getting tripped up by the prev->next pointer being null instead of pointing
    > > > to the current list entry while walking the i_wb_list
    > > >
    > > > Call traces are slightly different each time, but all end up walking i_wb_list
    > > > in dput -> d_kill -> i_put -> evict -> inode_wb_list_del
    > > >
    > > > What protects that list ? It looks to be just bdi->wb.list_lock ?
    > > >
    > > >
    > > > full reports at:
    > > > https://bugzilla.redhat.com/show_bug.cgi?id=784741
    > > > https://bugzilla.redhat.com/show_bug.cgi?id=799229
    > > > https://bugzilla.redhat.com/show_bug.cgi?id=799692
    > > Hum, interesting! I'd guess this might be caused by f758eeab - adding
    > > Fengguang and Christoph to CC. But I'm really failing to see how this could
    > > happen but interesting thing is that in two of the three cases the files
    > > are on virtual filesystems (once cgroup, once sysfs). These both use
    > > noop_backing_dev_info.
    >
    > sysfs/cgroup forgot to init inode->i_wb_list?
    Umm, it's not *that* simple I'd say. E.g. sysfs doesn't provide
    alloc_inode() method so we use inode_cachep for allocations. And that cache
    is configured to use inode_init_once().

    Also note that the error message is:
    list_del corruption. prev->next should be ffff8801c2f41b18, but was (null)

    Which means that our inode had correct i_wb_list.prev but the previous
    inode had NULL in i_wb_list.next. But that means that both inodes were
    linked into the list at some point. So it does not seem like an
    initialization issue to me...

    Honza

    > --- linux.orig/fs/inode.c 2012-02-22 19:20:48.374799955 -0800
    > +++ linux/fs/inode.c 2012-03-06 23:11:29.133899478 -0800
    > @@ -193,6 +193,7 @@ int inode_init_always(struct super_block
    > inode->i_private = NULL;
    > inode->i_mapping = mapping;
    > INIT_LIST_HEAD(&inode->i_dentry); /* buggered by rcu freeing */
    > + INIT_LIST_HEAD(&inode->i_wb_list);
    > #ifdef CONFIG_FS_POSIX_ACL
    > inode->i_acl = inode->i_default_acl = ACL_NOT_CACHED;
    > #endif


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