Messages in this thread | | | Date | Fri, 5 Nov 2021 09:21:08 -0400 | From | Steven Rostedt <> | Subject | Re: [syzbot] possible deadlock in ktime_get_coarse_ts64 |
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On Fri, 05 Nov 2021 14:10:21 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:
> > -> #0 (tk_core.seq.seqcount){----}-{0:0}: > > check_prev_add kernel/locking/lockdep.c:3051 [inline] > > check_prevs_add kernel/locking/lockdep.c:3174 [inline] > > validate_chain+0x1dfb/0x8240 kernel/locking/lockdep.c:3789 > > __lock_acquire+0x1382/0x2b00 kernel/locking/lockdep.c:5015 > > lock_acquire+0x19f/0x4d0 kernel/locking/lockdep.c:5625 > > seqcount_lockdep_reader_access+0xfe/0x230 include/linux/seqlock.h:103 > > ktime_get_coarse_ts64+0x25/0x110 kernel/time/timekeeping.c:2255 > > ktime_get_coarse include/linux/timekeeping.h:120 [inline] > > ktime_get_coarse_ns include/linux/timekeeping.h:126 [inline] > > --> this call is invalid > > > ____bpf_ktime_get_coarse_ns kernel/bpf/helpers.c:173 [inline] > > bpf_ktime_get_coarse_ns+0x7e/0x130 kernel/bpf/helpers.c:171 > > bpf_prog_a99735ebafdda2f1+0x10/0xb50 > > bpf_dispatcher_nop_func include/linux/bpf.h:721 [inline] > > __bpf_prog_run include/linux/filter.h:626 [inline] > > bpf_prog_run include/linux/filter.h:633 [inline] > > BPF_PROG_RUN_ARRAY include/linux/bpf.h:1294 [inline] > > trace_call_bpf+0x2cf/0x5d0 kernel/trace/bpf_trace.c:127 > > perf_trace_run_bpf_submit+0x7b/0x1d0 kernel/events/core.c:9708 > > perf_trace_lock+0x37c/0x440 include/trace/events/lock.h:39 > > trace_lock_release+0x128/0x150 include/trace/events/lock.h:58 > > Timestamps from within a tracepoint can only be taken with: > > 1) jiffies > 2) sched_clock() > 3) ktime_get_*_fast_ns()
This is why we have tracing clocks:
include/linux/trace_clock.h:
trace_clock_local() trace_clock() trace_clock_jiffies() trace_clock_global() trace_clock_counter()
And perf uses its own clock, which is either local_clock() or one of the ktime_get_*_ns() clocks.
-- Steve
> > Those are NMI safe and can be invoked from anywhere. > > All other time getters which have to use the timekeeping seqcount > protection are prone to live locks and _cannot_ be used from > tracepoints ever. > > This restriction exists since day one of tracepoints and is not > magically going away for BPF.
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