On Wed, Apr 04, 2018 at 09:35:46AM -0700, Mike Kravetz wrote:
> Running with this XArray series on top of next-20180329 consistently 'hangs'
> on shutdown looping (?forever?) in tag_pages_for_writeback/xas_for_each_tag.
> All I have to do is make sure there is some activity on the ext4 fs before
> shutdown. Not sure if this is a 'next-20180329' issue or XArray issue.
> But the fact that we are looping in xas_for_each_tag looks suspicious.
Thanks for your help debugging this! Particularly collecting the xa_dump.
I got bit by the undefined behaviour of shifting by BITS_PER_LONG,
but of course it was subtle.
The userspace testing framework wasn't catching this for a couple of
reasons; I'll work on making sure it catches this kind of thing in
the future.
I'll fold this in and post a v11 later this week or early next week.
diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index eac04922eba2..f5b7e507a86f 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -904,9 +929,12 @@ static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
if (advance)
offset++;
if (XA_CHUNK_SIZE == BITS_PER_LONG) {
- unsigned long data = *addr & (~0UL << offset);
- if (data)
- return __ffs(data);
+ if (offset < XA_CHUNK_SIZE) {
+ unsigned long data = *addr & (~0UL << offset);
+
+ if (data)
+ return __ffs(data);
+ }
return XA_CHUNK_SIZE;
}
