[lustre-devel] Apparent bug in LU-8130 ptlrpc: convert conn_hash to rhashtable / Linux-commit: ac2370ac2bc5215daf78546cd8d925510065bb7f

NeilBrown neilb at suse.com
Tue Nov 6 14:23:32 PST 2018

On Tue, Nov 06 2018, Patrick Farrell wrote:

> Neil, James,
> Sent this earlier, but with nasty formatting that got it rejected.  Whoops.
> It looks like the patch we landed as:
> LU-8130 ptlrpc: convert conn_hash to rhashtable
> Linux has a resizeable hashtable implementation in lib,
> so we should use that instead of having one in libcfs.
> This patch converts the ptlrpc conn_hash to use rhashtable.
> In the process we gain lockless lookup.
> As connections are never deleted until the hash table is destroyed,
> there is no need to count the reference in the hash table. There
> is also no need to enable automatic_shrinking.
> Linux-commit: ac2370ac2bc5215daf78546cd8d925510065bb7f
> Introduced a bug.  Ihara-san opened something to track it here:
> https://jira.whamcloud.com/browse/LU-11624
> It’s a null pointer in nid_hash(); there are some more details at that link.

That commit changed ptlrpc_connection_get() so that it can return NULL.

This is actually one of the really annoying things about rhashtable.  I
cannot convince the maintainers that sometimes you need guaranteed
success (which is quite possible with a hash table).  They come from the
networking community were they drop packets on errors all the time - no
big deal.

Anyway, all users of ptlrpc_connection_get() in the client code (in
drivers/staging) test the return value against NULL, so this was safe.

In SFS lustre, the call in target_handle_connect() assigned the
result to export->exp_connection without checking against NULL.
That leads to the oops.

ptlrpc_connection_get() will return NULL when
rhashtable_lookup_get_insert_fast() returns an error.
The errors are mostly weird internal implementation details leaking out
the API.  A retry, possibly after a short delay, will usually succeed.
(see https://lwn.net/Articles/751974/, section "Failure during insertion)
If this happens a lot, it might suggest that the hash function is poor.

Ahhhh... this was built against a 3.10 kernel.
Prior to 4.6, hash_64() was not so much "poor" as "appalling".
See https://lwn.net/Articles/687494/

For SFS we should avoid using hash_64() (at least before 4.6) and
instead use something like
  hash_32((foo>>32) ^ hash_32(foo & 0xFFFFFFFF))


> We’re seeing it at Cray as well, when testing the current WhamCloud branch.
> Basically, when we fail over an MDS(/MDT) under load (ie with real activity on the file system) we hit this panic about 30-50% of the time right now.  I assume it’s possible on OSSes as well but we haven’t seen it there.
> I haven’t done any detailed investigation, but I thought I’d bring it to your attention.  Per Ihara-san in LU-11624, the crash does not happen without the commit listed above.
> • Patrick
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