sunrpc: fix race in cache cleanup causing stale nextcheck time

When cache cleanup runs concurrently with cache entry removal, a race
condition can occur that leads to incorrect nextcheck times. This can
delay cache cleanup for the cache_detail by up to 1800 seconds:

1. cache_clean() sets nextcheck to current time plus 1800 seconds
2. While scanning a non-empty bucket, concurrent cache entry removal can
empty that bucket
3. cache_clean() finds no cache entries in the now-empty bucket to update
the nextcheck time
4. This maybe delays the next scan of the cache_detail by up to 1800
seconds even when it should be scanned earlier based on remaining
entries

Fix this by moving the hash_lock acquisition earlier in cache_clean().
This ensures bucket emptiness checks and nextcheck updates happen
atomically, preventing the race between cleanup and entry removal.

Signed-off-by: Long Li <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

net/sunrpc/cache.c

···
 		}
 	}
 
+	spin_lock(&current_detail->hash_lock);
+
 	/* find a non-empty bucket in the table */
-	while (current_detail &&
-	       current_index < current_detail->hash_size &&
+	while (current_index < current_detail->hash_size &&
 	       hlist_empty(&current_detail->hash_table[current_index]))
 		current_index++;
 
 	/* find a cleanable entry in the bucket and clean it, or set to next bucket */
-
-	if (current_detail && current_index < current_detail->hash_size) {
+	if (current_index < current_detail->hash_size) {
 		struct cache_head *ch = NULL;
 		struct cache_detail *d;
 		struct hlist_head *head;
 		struct hlist_node *tmp;
 
-		spin_lock(&current_detail->hash_lock);
-
 		/* Ok, now to clean this strand */
-
 		head = &current_detail->hash_table[current_index];
 		hlist_for_each_entry_safe(ch, tmp, head, cache_list) {
 			if (current_detail->nextcheck > ch->expiry_time)
···
 		spin_unlock(&cache_list_lock);
 		if (ch)
 			sunrpc_end_cache_remove_entry(ch, d);
-	} else
+	} else {
+		spin_unlock(&current_detail->hash_lock);
 		spin_unlock(&cache_list_lock);
+	}
 
 	return rv;
 }