Re: [tip:timers/core] [timekeeping] 5aa6c43eca: BUG:KCSAN:data-race_in_timekeeping_debug_get_ns/timekeeping_update_from_shadow

[John Stultz]

On Wed, Oct 30, 2024 at 1:50 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Wed, Oct 30 2024 at 13:47, kernel test robot wrote:
> > this is another report about BUG:KCSAN, the change does not introduce new KCSAN
> > issue, but causes stats changes as below.
> >
> > [   70.265411][    C1] BUG: KCSAN: data-race in timekeeping_debug_get_ns / timekeeping_update_from_shadow
> > [   70.265430][    C1]
> > [   70.265433][    C1] write to 0xffffffff8483fef8 of 296 bytes by interrupt on cpu 0:
> > [ 70.265440][ C1] timekeeping_update_from_shadow+0x8e/0x140
> > [ 70.265452][ C1] timekeeping_advance (kernel/time/timekeeping.c:2394)
> > [ 70.265462][ C1] update_wall_time (kernel/time/timekeeping.c:2403)
>
> timekeeping_update_from_shadow() holds the sequence count write.
>
> > [ 70.265642][ C1] timekeeping_debug_get_ns (kernel/time/timekeeping.c:415 kernel/time/timekeeping.c:399 kernel/time/timekeeping.c:307)
> > [ 70.265653][ C1] ktime_get (kernel/time/timekeeping.c:431 (discriminator 4) kernel/time/timekeeping.c:897 (discriminator 4))
> > [ 70.265660][ C1] tick_nohz_lowres_handler (kernel/time/tick-sched.c:220 kernel/time/tick-sched.c:290 kernel/time/tick-sched.c:1486)
>
> ktime_get()
>
>         do {
>            seq = read_seqcount_begin(&tk_core.seq);
>            timekeeping_debug_get_ns();
>         } while (read_seqcount_retry(&tk_core.seq, seq));
>
> So this should be safe against concurreny. I assume the issue here is
> that timekeeping_debug_get_ns() has a nested
>
>         do {
>            seq = read_seqcount_begin(&tk_core.seq);
>            ....
>         } while (read_seqcount_retry(&tk_core.seq, seq));
>
> inside. Which is still correct, but confuses KCSAN. Marco?
>
> But that aside, since 135225a363ae timekeeping_cycles_to_ns() is fully
> overflow protected and unconditionally handles negative motion (before
> it was x86 only), the value of timekeeping_debug_get_ns() becomes
> questionable.
>
> I'm leaning towards removing it completely.
>
> John?

Yeah. I could be wrong, but I'm not sure of anyone beyond myself that
has really utilized the TIMEKEEPING_DEBUG logic (and I've not enabled
it myself in a few years). I don't think we've had any problem reports
from it either.

So no objection from me.

thanks
-john