bpf: Update the comments in maybe_wait_bpf_programs()

Since commit 638e4b8 ("bpf: Allows per-cpu maps and map-in-map in
sleepable programs"), sleepable BPF program can also use map-in-map, but
maybe_wait_bpf_programs() doesn't handle it accordingly. The main reason
is that using synchronize_rcu_tasks_trace() to wait for the completions
of these sleepable BPF programs may incur a very long delay and
userspace may think it is hung, so the wait for sleepable BPF programs
is skipped. Update the comments in maybe_wait_bpf_programs() to reflect
the reason.

Signed-off-by: Hou Tao <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Hou Tao

kernel/bpf/syscall.c

@@ -142,9 +142,13 @@ static u32 bpf_map_value_size(const struct bpf_map *map)
 
 static void maybe_wait_bpf_programs(struct bpf_map *map)
 {
-	/* Wait for any running BPF programs to complete so that
-	 * userspace, when we return to it, knows that all programs
-	 * that could be running use the new map value.
+	/* Wait for any running non-sleepable BPF programs to complete so that
+	 * userspace, when we return to it, knows that all non-sleepable
+	 * programs that could be running use the new map value. For sleepable
+	 * BPF programs, synchronize_rcu_tasks_trace() should be used to wait
+	 * for the completions of these programs, but considering the waiting
+	 * time can be very long and userspace may think it will hang forever,
+	 * so don't handle sleepable BPF programs now.
 	 */
 	if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS ||
 	    map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS)