Re: [syzbot] possible deadlock in ktime_get_coarse_ts64

Steven Rostedt
2021-11-05T13:21:15Z

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.