bpf: cpumap: Fix possible rcpu kthread hung

Fix the following cpumap kthread hung. The issue is currently occurring
when __cpu_map_load_bpf_program fails (e.g if the bpf prog has not
BPF_XDP_CPUMAP as expected_attach_type)

$./test_progs -n 101
101/1 cpumap_with_progs:OK
101 xdp_cpumap_attach:OK
Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
[  369.996478] INFO: task cpumap/0/map:7:205 blocked for more than 122 seconds.
[  369.998463]       Not tainted 5.8.0-rc4-01472-ge57892f5 #212
[  370.000102] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  370.001918] cpumap/0/map:7  D    0   205      2 0x00004000
[  370.003228] Call Trace:
[  370.003930]  __schedule+0x5c7/0xf50
[  370.004901]  ? io_schedule_timeout+0xb0/0xb0
[  370.005934]  ? static_obj+0x31/0x80
[  370.006788]  ? mark_held_locks+0x24/0x90
[  370.007752]  ? cpu_map_bpf_prog_run_xdp+0x6c0/0x6c0
[  370.008930]  schedule+0x6f/0x160
[  370.009728]  schedule_preempt_disabled+0x14/0x20
[  370.010829]  kthread+0x17b/0x240
[  370.011433]  ? kthread_create_worker_on_cpu+0xd0/0xd0
[  370.011944]  ret_from_fork+0x1f/0x30
[  370.012348]
               Showing all locks held in the system:
[  370.013025] 1 lock held by khungtaskd/33:
[  370.013432]  #0: ffffffff82b24720 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x28/0x1c3

[  370.014461] =============================================

Fixes: 92164774 ("bpf: cpumap: Add the possibility to attach an eBPF program to cpumap")
Reported-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/e54f2aabf959f298939e5507b09c48f8c2e380be.1595170625.git.lorenzo@kernel.org

kernel/bpf/cpumap.c

@@ -453,24 +453,27 @@ __cpu_map_entry_alloc(struct bpf_cpumap_val *value, u32 cpu, int map_id)
 	rcpu->map_id = map_id;
 	rcpu->value.qsize  = value->qsize;
 
+	if (fd > 0 && __cpu_map_load_bpf_program(rcpu, fd))
+		goto free_ptr_ring;
+
 	/* Setup kthread */
 	rcpu->kthread = kthread_create_on_node(cpu_map_kthread_run, rcpu, numa,
 					       "cpumap/%d/map:%d", cpu, map_id);
 	if (IS_ERR(rcpu->kthread))
-		goto free_ptr_ring;
+		goto free_prog;
 
 	get_cpu_map_entry(rcpu); /* 1-refcnt for being in cmap->cpu_map[] */
 	get_cpu_map_entry(rcpu); /* 1-refcnt for kthread */
 
-	if (fd > 0 && __cpu_map_load_bpf_program(rcpu, fd))
-		goto free_ptr_ring;
-
 	/* Make sure kthread runs on a single CPU */
 	kthread_bind(rcpu->kthread, cpu);
 	wake_up_process(rcpu->kthread);
 
 	return rcpu;
 
+free_prog:
+	if (rcpu->prog)
+		bpf_prog_put(rcpu->prog);
 free_ptr_ring:
 	ptr_ring_cleanup(rcpu->queue, NULL);
 free_queue: