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Task can be interrupted by >>>>>>>>> NMI, if the same map bucket was locked, there will be a deadlock. >>>>>>>>> >>>>>>>>> * map max_entries is 2. >>>>>>>>> * NMI using key 4 and Task context using key 20. >>>>>>>>> * so same bucket index but map_locked index is different. >>>>>>>>> >>>>>>>>> The selftest use perf to produce the NMI and fentry nmi_handle. >>>>>>>>> Note that bpf_overflow_handler checks bpf_prog_active, but in bpf update >>>>>>>>> map syscall increase this counter in bpf_disable_instrumentation. >>>>>>>>> Then fentry nmi_handle and update hash map will reproduce the issue. >>> SNIP >>>>>>>>> diff --git a/tools/testing/selftests/bpf/progs/htab_deadlock.c >>>>>>>>> b/tools/testing/selftests/bpf/progs/htab_deadlock.c >>>>>>>>> new file mode 100644 >>>>>>>>> index 000000000000..d394f95e97c3 >>>>>>>>> --- /dev/null >>>>>>>>> +++ b/tools/testing/selftests/bpf/progs/htab_deadlock.c >>>>>>>>> @@ -0,0 +1,32 @@ >>>>>>>>> +// SPDX-License-Identifier: GPL-2.0 >>>>>>>>> +/* Copyright (c) 2022 DiDi Global Inc. */ >>>>>>>>> +#include >>>>>>>>> +#include >>>>>>>>> +#include >>>>>>>>> + >>>>>>>>> +char _license[] SEC("license") = "GPL"; >>>>>>>>> + >>>>>>>>> +struct { >>>>>>>>> + __uint(type, BPF_MAP_TYPE_HASH); >>>>>>>>> + __uint(max_entries, 2); >>>>>>>>> + __uint(map_flags, BPF_F_ZERO_SEED); >>>>>>>>> + __type(key, unsigned int); >>>>>>>>> + __type(value, unsigned int); >>>>>>>>> +} htab SEC(".maps"); >>>>>>>>> + >>>>>>>>> +/* nmi_handle on x86 platform. If changing keyword >>>>>>>>> + * "static" to "inline", this prog load failed. */ >>>>>>>>> +SEC("fentry/nmi_handle") >>>>>>>> >>>>>>>> The above comment is not what I mean. In arch/x86/kernel/nmi.c, >>>>>>>> we have >>>>>>>> static int nmi_handle(unsigned int type, struct pt_regs *regs) >>>>>>>> { >>>>>>>> ... >>>>>>>> } >>>>>>>> ... >>>>>>>> static noinstr void default_do_nmi(struct pt_regs *regs) >>>>>>>> { >>>>>>>> ... >>>>>>>> handled = nmi_handle(NMI_LOCAL, regs); >>>>>>>> ... >>>>>>>> } >>>>>>>> >>>>>>>> Since nmi_handle is a static function, it is possible that >>>>>>>> the function might be inlined in default_do_nmi by the >>>>>>>> compiler. If this happens, fentry/nmi_handle will not >>>>>>>> be triggered and the test will pass. >>>>>>>> >>>>>>>> So I suggest to change the comment to >>>>>>>> nmi_handle() is a static function and might be >>>>>>>> inlined into its caller. If this happens, the >>>>>>>> test can still pass without previous kernel fix. >>>>>>> >>>>>>> It's worse than this. >>>>>>> fentry is buggy. >>>>>>> We shouldn't allow attaching fentry to: >>>>>>> NOKPROBE_SYMBOL(nmi_handle); >>>>>> >>>>>> Okay, I see. Looks we should prevent fentry from >>>>>> attaching any NOKPROBE_SYMBOL functions. >>>>>> >>>>>> BTW, I think fentry/nmi_handle can be replaced with >>>>>> tracepoint nmi/nmi_handler. it is more reliable >>>>> The tracepoint will not reproduce the deadlock(we have discussed v2). >>>>> If it's not easy to complete a test for this case, should we drop this >>>>> testcase patch? or fentry the nmi_handle and update the comments. >>>> >>>> could we use a softirq perf event (timer), e.g., >>>> >>>> struct perf_event_attr attr = { >>>> .sample_period = 1, >>>> .type = PERF_TYPE_SOFTWARE, >>>> .config = PERF_COUNT_SW_CPU_CLOCK, >>>> }; >>>> >>>> then you can attach function hrtimer_run_softirq (not tested) or >>>> similar functions? >>> The context will be a hard-irq context, right ? Because htab_lock_bucket() has >>> already disabled hard-irq on current CPU, so the dead-lock will be impossible. >> >> Okay, I see. soft-irq doesn't work. The only thing it works is nmi since >> it is non-masking. >> >>>> >>>> I suspect most (if not all) functions in nmi path cannot >>>> be kprobe'd. >>> It seems that perf_event_nmi_handler() is also nokprobe function. However I >>> think we could try its callees (e.g., x86_pmu_handle_irq or perf_event_overflow). >> >> If we can find a function in nmi handling path which is not marked as >> nonkprobe, sure, we can use that function for fentry. > I think perf_event_overflow is ok. > [ 93.233093] dump_stack_lvl+0x57/0x81 > [ 93.233098] lock_acquire+0x1f4/0x29a > [ 93.233103] ? htab_lock_bucket+0x61/0x6c > [ 93.233108] _raw_spin_lock_irqsave+0x43/0x7f > [ 93.233111] ? htab_lock_bucket+0x61/0x6c > [ 93.233114] htab_lock_bucket+0x61/0x6c > [ 93.233118] htab_map_update_elem+0x11e/0x220 > [ 93.233124] bpf_prog_df326439468c24a9_bpf_prog1+0x41/0x45 > [ 93.233137] bpf_trampoline_6442478975_0+0x48/0x1000 > [ 93.233144] perf_event_overflow+0x5/0x15 > [ 93.233149] handle_pmi_common+0x1ad/0x1f0 > [ 93.233166] intel_pmu_handle_irq+0x136/0x18a > [ 93.233170] perf_event_nmi_handler+0x28/0x47 > [ 93.233176] nmi_handle+0xb8/0x254 > [ 93.233182] default_do_nmi+0x3d/0xf6 > [ 93.233187] exc_nmi+0xa1/0x109 > [ 93.233191] end_repeat_nmi+0x16/0x67 sounds good. I agree fentry/perf_event_overflow can be used for the test. >>>> >>>>>> and won't be impacted by potential NOKPROBE_SYMBOL >>>>>> issues. >>>>> >>>>> >>>>> >>>> . >>> > > >