net/xdp: Fix suspicious RCU usage warning

Fix the warning below by calling rhashtable_lookup_fast.
Also, make some code movements for better quality and human
readability.

[  342.450870] WARNING: suspicious RCU usage
[  342.455856] 4.18.0-rc2+ #17 Tainted: G           O
[  342.462210] -----------------------------
[  342.467202] ./include/linux/rhashtable.h:481 suspicious rcu_dereference_check() usage!
[  342.476568]
[  342.476568] other info that might help us debug this:
[  342.476568]
[  342.486978]
[  342.486978] rcu_scheduler_active = 2, debug_locks = 1
[  342.495211] 4 locks held by modprobe/3934:
[  342.500265]  #0: 00000000e23116b2 (mlx5_intf_mutex){+.+.}, at:
mlx5_unregister_interface+0x18/0x90 [mlx5_core]
[  342.511953]  #1: 00000000ca16db96 (rtnl_mutex){+.+.}, at: unregister_netdev+0xe/0x20
[  342.521109]  #2: 00000000a46e2c4b (&priv->state_lock){+.+.}, at: mlx5e_close+0x29/0x60
[mlx5_core]
[  342.531642]  #3: 0000000060c5bde3 (mem_id_lock){+.+.}, at: xdp_rxq_info_unreg+0x93/0x6b0
[  342.541206]
[  342.541206] stack backtrace:
[  342.547075] CPU: 12 PID: 3934 Comm: modprobe Tainted: G           O      4.18.0-rc2+ #17
[  342.556621] Hardware name: Dell Inc. PowerEdge R730/0H21J3, BIOS 1.5.4 10/002/2015
[  342.565606] Call Trace:
[  342.568861]  dump_stack+0x78/0xb3
[  342.573086]  xdp_rxq_info_unreg+0x3f5/0x6b0
[  342.578285]  ? __call_rcu+0x220/0x300
[  342.582911]  mlx5e_free_rq+0x38/0xc0 [mlx5_core]
[  342.588602]  mlx5e_close_channel+0x20/0x120 [mlx5_core]
[  342.594976]  mlx5e_close_channels+0x26/0x40 [mlx5_core]
[  342.601345]  mlx5e_close_locked+0x44/0x50 [mlx5_core]
[  342.607519]  mlx5e_close+0x42/0x60 [mlx5_core]
[  342.613005]  __dev_close_many+0xb1/0x120
[  342.617911]  dev_close_many+0xa2/0x170
[  342.622622]  rollback_registered_many+0x148/0x460
[  342.628401]  ? __lock_acquire+0x48d/0x11b0
[  342.633498]  ? unregister_netdev+0xe/0x20
[  342.638495]  rollback_registered+0x56/0x90
[  342.643588]  unregister_netdevice_queue+0x7e/0x100
[  342.649461]  unregister_netdev+0x18/0x20
[  342.654362]  mlx5e_remove+0x2a/0x50 [mlx5_core]
[  342.659944]  mlx5_remove_device+0xe5/0x110 [mlx5_core]
[  342.666208]  mlx5_unregister_interface+0x39/0x90 [mlx5_core]
[  342.673038]  cleanup+0x5/0xbfc [mlx5_core]
[  342.678094]  __x64_sys_delete_module+0x16b/0x240
[  342.683725]  ? do_syscall_64+0x1c/0x210
[  342.688476]  do_syscall_64+0x5a/0x210
[  342.693025]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 8d5d885 ("xdp: rhashtable with allocator ID to pointer mapping")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/xdp.c

@@ -98,23 +98,15 @@ static void __xdp_rxq_info_unreg_mem_model(struct xdp_rxq_info *xdp_rxq)
 {
 	struct xdp_mem_allocator *xa;
 	int id = xdp_rxq->mem.id;
-	int err;
 
 	if (id == 0)
 		return;
 
 	mutex_lock(&mem_id_lock);
 
-	xa = rhashtable_lookup(mem_id_ht, &id, mem_id_rht_params);
-	if (!xa) {
-		mutex_unlock(&mem_id_lock);
-		return;
-	}
-
-	err = rhashtable_remove_fast(mem_id_ht, &xa->node, mem_id_rht_params);
-	WARN_ON(err);
-
-	call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
+	xa = rhashtable_lookup_fast(mem_id_ht, &id, mem_id_rht_params);
+	if (xa && !rhashtable_remove_fast(mem_id_ht, &xa->node, mem_id_rht_params))
+		call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
 
 	mutex_unlock(&mem_id_lock);
 }