On 07/17/2018 06:47 PM, Alexei Starovoitov wrote:
> On Tue, Jul 17, 2018 at 06:10:38PM +0300, Tariq Toukan wrote:
>> Fix the warning below by calling rhashtable_lookup under
>> RCU read lock.
>>
>> [ 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: 8d5d88527587 ("xdp: rhashtable with allocator ID to pointer mapping")
>> Signed-off-by: Tariq Toukan <tar...@mellanox.com>
>> Cc: Jesper Dangaard Brouer <bro...@redhat.com>
>> ---
>> net/core/xdp.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>> index 9d1f22072d5d..c20fefbfb76c 100644
>> --- a/net/core/xdp.c
>> +++ b/net/core/xdp.c
>> @@ -102,7 +102,9 @@ static void __xdp_rxq_info_unreg_mem_model(struct
>> xdp_rxq_info *xdp_rxq)
>>
>> mutex_lock(&mem_id_lock);
>>
>> + rcu_read_lock();
>> xa = rhashtable_lookup(mem_id_ht, &id, mem_id_rht_params);
>> + rcu_read_unlock();
>> if (!xa) {
>
> if it's an actual bug rcu_read_unlock seems to be misplaced.
> It silences the warn, but rcu section looks wrong.
I think that whole piece in __xdp_rxq_info_unreg_mem_model() should be:
mutex_lock(&mem_id_lock);
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) ==
0)
call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
mutex_unlock(&mem_id_lock);
Technically the RCU read side plus rhashtable_lookup() is the same, but lets
use proper api. From the doc (https://lwn.net/Articles/751374/) object removal
is wrapped around the RCU read side additionally, but in our case we're behind
mem_id_lock for insertion/removal serialization.
Cheers,
Daniel
