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bpf: Update the comments in maybe_wait_bpf_programs()

Since commit 638e4b825d52 ("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 <houtao1@huawei.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231211083447.1921178-1-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

authored by

Hou Tao and committed by
Alexei Starovoitov 2a0c6b41 b13cddf6

+7 -3
+7 -3
kernel/bpf/syscall.c
··· 142 142 143 143 static void maybe_wait_bpf_programs(struct bpf_map *map) 144 144 { 145 - /* Wait for any running BPF programs to complete so that 146 - * userspace, when we return to it, knows that all programs 147 - * that could be running use the new map value. 145 + /* Wait for any running non-sleepable BPF programs to complete so that 146 + * userspace, when we return to it, knows that all non-sleepable 147 + * programs that could be running use the new map value. For sleepable 148 + * BPF programs, synchronize_rcu_tasks_trace() should be used to wait 149 + * for the completions of these programs, but considering the waiting 150 + * time can be very long and userspace may think it will hang forever, 151 + * so don't handle sleepable BPF programs now. 148 152 */ 149 153 if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS || 150 154 map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS)