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

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Message-ID: <87o730wnl3.ffs@tglx>
Date: Thu, 31 Oct 2024 11:53:12 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: John Stultz <jstultz@...gle.com>
Cc: kernel test robot <oliver.sang@...el.com>, Anna-Maria Behnsen
 <anna-maria@...utronix.de>, oe-lkp@...ts.linux.dev, lkp@...el.com,
 linux-kernel@...r.kernel.org, x86@...nel.org, Marco Elver
 <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Frederic
 Weisbecker <frederic@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Stephen Boyd <sboyd@...nel.org>
Subject: Re: [tip:timers/core] [timekeeping] 5aa6c43eca:
 BUG:KCSAN:data-race_in_timekeeping_debug_get_ns/timekeeping_update_from_shadow

On Thu, Oct 31 2024 at 00:35, Thomas Gleixner wrote:

> On Wed, Oct 30 2024 at 15:16, John Stultz wrote:
>> On Wed, Oct 30, 2024 at 1:50 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>> 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.
>
> The question is whether we want to preserve the remaining 'offset'
> check.  I.e. either discard it or make it unconditional? It's cheep now.

Nah. It's irrelevant now too. Before the time getters gained the math
overflow handling this definitely was a problem, but now it's just
noise. timekeeping_advance() can handle that case correctly.

Thanks,

        tglx