[v2 PATCH] rhashtable: Fix rhashtable_try_insert test

[Herbert Xu <herbert@gondor.apana.org.au>]

On Fri, Jan 10, 2025 at 04:59:28PM +0000, Michael Kelley wrote:
>
> This patch fixes the problem I saw with VMs in the Azure cloud.  Thanks!

Sorry, but the test on data is needed after all (it was just
buggy).  Otherwise we will break rhlist.  So please test this
patch instead.

---8<---
The test on whether rhashtable_insert_one did an insertion relies
on the value returned by rhashtable_lookup_one.  Unfortunately that
value is overwritten after rhashtable_insert_one returns.  Fix this
by saving the old value.

Also simplify the test as only data == NULL matters.

Reported-by: Michael Kelley <mhklinux@outlook.com>
Fixes: e1d3422c95f0 ("rhashtable: Fix potential deadlock by moving schedule_work outside lock")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index bf956b85455a..e36b36f3146d 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -611,17 +611,20 @@ static void *rhashtable_try_insert(struct rhashtable *ht, const void *key,
 			new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
 			data = ERR_PTR(-EAGAIN);
 		} else {
+			void *odata;
+
 			flags = rht_lock(tbl, bkt);
 			data = rhashtable_lookup_one(ht, bkt, tbl,
 						     hash, key, obj);
 			new_tbl = rhashtable_insert_one(ht, bkt, tbl,
 							hash, obj, data);
+			odata = data;
 			if (PTR_ERR(new_tbl) != -EEXIST)
 				data = ERR_CAST(new_tbl);
 
 			rht_unlock(tbl, bkt, flags);
 
-			if (PTR_ERR(data) == -ENOENT && !new_tbl) {
+			if (odata && !new_tbl) {
 				atomic_inc(&ht->nelems);
 				if (rht_grow_above_75(ht, tbl))
 					schedule_work(&ht->run_work);
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt