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

[James Simmons]

> > 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))

The libcfs hash is not much better :-( 

I have a purposed fix. Let me know if its correct. Thank you.

>From 32df018c1af8fdbc833c537aafeea081ab3d8d7e Mon Sep 17 00:00:00 2001
From: James Simmons <uja.ornl at yahoo.com>
Date: Wed, 7 Nov 2018 14:06:37 -0500
Subject: [PATCH] LU-11624 ptlrpc: handle no ptlrpc connection case

With the move of ptlrpc connections hash to rhashtable it is now
possible for ptlrpc_connection_get() to return NULL. The reason
this can happen is due to conditions. One is that the hash has no
more free slots which results in an E2BIG error. By default the
maximum size for rhashtables is 2^31 so this is very unlikely
going to happen. The second case returns a -ENOMEM which can
happen when the bucket can be resized. For this case test the
return value of rhashtable_lookup_get_insert_fast() and if it is
-ENOMEM try again inserting the new connection into the hash
table. Just as an extra level of protection against a failed
connection attempt in target_handle_connect() exit out with
-ENOTCONN.

Change-Id: I0537564ba544ed06be42ba243606a884a1290f20
Signed-off-by: James Simmons <uja.ornl at yahoo.com>
---
 lustre/ldlm/ldlm_lib.c     | 3 +++
 lustre/ptlrpc/connection.c | 6 +++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/lustre/ldlm/ldlm_lib.c b/lustre/ldlm/ldlm_lib.c
index de66aa3..463c6c6 100644
--- a/lustre/ldlm/ldlm_lib.c
+++ b/lustre/ldlm/ldlm_lib.c
@@ -1349,6 +1349,9 @@ dont_check_exports:
 	export->exp_connection = ptlrpc_connection_get(req->rq_peer,
 						       req->rq_self,
 						       &cluuid);
+	if (!export->exp_connection)
+		GOTO(out, rc = -ENOTCONN);
+
 	if (hlist_unhashed(&export->exp_nid_hash))
 		cfs_hash_add(export->exp_obd->obd_nid_hash,
 			     &export->exp_connection->c_peer.nid,
diff --git a/lustre/ptlrpc/connection.c b/lustre/ptlrpc/connection.c
index 21a24a2..045b3ee 100644
--- a/lustre/ptlrpc/connection.c
+++ b/lustre/ptlrpc/connection.c
@@ -103,13 +103,17 @@ ptlrpc_connection_get(struct lnet_process_id peer, lnet_nid_t self,
 	 * connection.	The object which exists in the hash will be
 	 * returned,otherwise NULL is returned on success.
 	 */
+try_again:
 	conn2 = rhashtable_lookup_get_insert_fast(&conn_hash, &conn->c_hash,
 						  conn_hash_params);
 	if (conn2) {
 		/* insertion failed */
 		OBD_FREE_PTR(conn);
-		if (IS_ERR(conn2))
+		if (IS_ERR(conn2)) {
+			if (PTR_ERR(conn2) == -ENOMEM)
+				goto try_again;
 			return NULL;
+		}
 		conn = conn2;
 		ptlrpc_connection_addref(conn);
 	}
-- 
1.8.3.1