On Wed 20-06-18 21:29:12, Matthew Wilcox wrote:
> On Thu, Jun 21, 2018 at 11:32:45AM +0800, Jia-Ju Bai wrote:
> > The kernel may sleep with holding a spinlock.
> > The function call paths (from bottom to top) in Linux-4.16.7 are:
> >
> > [FUNC] kmem_cache_alloc(GFP_KERNEL)
> > fs/notify/mark.c, 439:
> > kmem_cache_alloc in fsnotify_attach_connector_to_object
> > fs/notify/mark.c, 520:
> > fsnotify_attach_connector_to_object in fsnotify_add_mark_list
> > fs/notify/mark.c, 590:
> > fsnotify_add_mark_list in fsnotify_add_mark_locked
> > kernel/audit_tree.c, 437:
> > fsnotify_add_mark_locked in tag_chunk
> > kernel/audit_tree.c, 423:
> > spin_lock in tag_chunk
>
> There are several locks here; your report would be improved by saying
> which one is the problem. I'm assuming it's old_entry->lock.
>
> spin_lock(&old_entry->lock);
> ...
> if (fsnotify_add_inode_mark_locked(chunk_entry,
> old_entry->connector->inode, 1)) {
> ...
> return fsnotify_add_mark_locked(mark, inode, NULL, allow_dups);
> ...
> ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups);
> ...
> if (inode)
> connp = &inode->i_fsnotify_marks;
> conn = fsnotify_grab_connector(connp);
> if (!conn) {
> err = fsnotify_attach_connector_to_object(connp, inode, mnt);
>
> It seems to me that this is safe because old_entry is looked up from
> fsnotify_find_mark, and it can't be removed while its lock is held.
> Therefore there's always a 'conn' returned from fsnotify_grab_connector(),
> and so this path will never be taken.
>
> But this code path is confusing to me, and I could be wrong. Jan, please
> confirm my analysis is correct?
Yes, you are correct. The presence of another mark in the list (and the
fact we pin it there using refcount & mark_mutex) guarantees we won't need
to allocate the connector. I agree the audit code's use of fsnotify would
deserve some cleanup.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
