Re: [net-next] bpf: avoid hashtab deadlock with try_lock

[Hou Tao <houtao1@huawei.com>]

Hi,

On 11/29/2022 5:55 AM, Hao Luo wrote:
> On Sun, Nov 27, 2022 at 7:15 PM Tonghao Zhang <xiangxia.m.yue@gmail.com> wrote:
> Hi Tonghao,
>
> With a quick look at the htab_lock_bucket() and your problem
> statement, I agree with Hou Tao that using hash &
> min(HASHTAB_MAP_LOCK_MASK, n_bucket - 1) to index in map_locked seems
> to fix the potential deadlock. Can you actually send your changes as
> v2 so we can take a look and better help you? Also, can you explain
> your solution in your commit message? Right now, your commit message
> has only a problem statement and is not very clear. Please include
> more details on what you do to fix the issue.
>
> Hao
It would be better if the test case below can be rewritten as a bpf selftests.
Please see comments below on how to improve it and reproduce the deadlock.
>
>> Hi
>> only a warning from lockdep.
Thanks for your details instruction.  I can reproduce the warning by using your
setup. I am not a lockdep expert, it seems that fixing such warning needs to set
different lockdep class to the different bucket. Because we use map_locked to
protect the acquisition of bucket lock, so I think we can define  lock_class_key
array in bpf_htab (e.g., lockdep_key[HASHTAB_MAP_LOCK_COUNT]) and initialize the
bucket lock accordingly.

>> 1. the kernel .config
>> #
>> # Debug Oops, Lockups and Hangs
>> #
>> CONFIG_PANIC_ON_OOPS=y
>> CONFIG_PANIC_ON_OOPS_VALUE=1
>> CONFIG_PANIC_TIMEOUT=0
>> CONFIG_LOCKUP_DETECTOR=y
>> CONFIG_SOFTLOCKUP_DETECTOR=y
>> # CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC is not set
>> CONFIG_HARDLOCKUP_DETECTOR_PERF=y
>> CONFIG_HARDLOCKUP_CHECK_TIMESTAMP=y
>> CONFIG_HARDLOCKUP_DETECTOR=y
>> CONFIG_BOOTPARAM_HARDLOCKUP_PANIC=y
>> CONFIG_DETECT_HUNG_TASK=y
>> CONFIG_DEFAULT_HUNG_TASK_TIMEOUT=120
>> # CONFIG_BOOTPARAM_HUNG_TASK_PANIC is not set
>> # CONFIG_WQ_WATCHDOG is not set
>> # CONFIG_TEST_LOCKUP is not set
>> # end of Debug Oops, Lockups and Hangs
>>
>> 2. bpf.c, the map size is 2.
>> struct {
>> __uint(type, BPF_MAP_TYPE_HASH);
Adding __uint(map_flags, BPF_F_ZERO_SEED); to ensure there will be no seed for
hash calculation, so we can use key=4 and key=20 to construct the case that
these two keys have the same bucket index but have different map_locked index.
>> __uint(max_entries, 2);
>> __uint(key_size, sizeof(unsigned int));
>> __uint(value_size, sizeof(unsigned int));
>> } map1 SEC(".maps");
>>
>> static int bpf_update_data()
>> {
>> unsigned int val = 1, key = 0;
key = 20
>>
>> return bpf_map_update_elem(&map1, &key, &val, BPF_ANY);
>> }
>>
>> SEC("kprobe/ip_rcv")
>> int bpf_prog1(struct pt_regs *regs)
>> {
>> bpf_update_data();
>> return 0;
>> }
kprobe on ip_rcv is unnecessary, you can just remove it.
>>
>> SEC("tracepoint/nmi/nmi_handler")
>> int bpf_prog2(struct pt_regs *regs)
>> {
>> bpf_update_data();
>> return 0;
>> }
Please use SEC("fentry/nmi_handle") instead of SEC("tracepoint") and unfold
bpf_update_data(), because the running of bpf program on tracepoint will be
blocked by bpf_prog_active which will be increased bpf_map_update_elem through
bpf_disable_instrumentation().
>>
>> char _license[] SEC("license") = "GPL";
>> unsigned int _version SEC("version") = LINUX_VERSION_CODE;
>>
>> 3. bpf loader.
>> #include "kprobe-example.skel.h"
>>
>> #include <unistd.h>
>> #include <errno.h>
>>
>> #include <bpf/bpf.h>
>>
>> int main()
>> {
>> struct kprobe_example *skel;
>> int map_fd, prog_fd;
>> int i;
>> int err = 0;
>>
>> skel = kprobe_example__open_and_load();
>> if (!skel)
>> return -1;
>>
>> err = kprobe_example__attach(skel);
>> if (err)
>> goto cleanup;
>>
>> /* all libbpf APIs are usable */
>> prog_fd = bpf_program__fd(skel->progs.bpf_prog1);
>> map_fd = bpf_map__fd(skel->maps.map1);
>>
>> printf("map_fd: %d\n", map_fd);
>>
>> unsigned int val = 0, key = 0;
>>
>> while (1) {
>> bpf_map_delete_elem(map_fd, &key);
No needed neither. Only do bpf_map_update_elem() is OK. Also change key=0 from
key=4, so it will have the same bucket index as key=20 but have different
map_locked index.
>> bpf_map_update_elem(map_fd, &key, &val, BPF_ANY);
>> }
Also need to pin the process on a specific CPU (e.g., CPU 0)
>>
>> cleanup:
>> kprobe_example__destroy(skel);
>> return err;
>> }
>>
>> 4. run the bpf loader and perf record for nmi interrupts.  the warming occurs
For perf event, you can reference prog_tests/find_vma.c on how to using
perf_event_open to trigger a perf nmi interrupt. The perf event also needs to
pin on a specific CPU as the caller of bpf_map_update_elem() does.

>>
>> --
>> Best regards, Tonghao
> .