linux-kernel - Re: [timekeeping] 3aeaac747d: PANIC:early

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Date:   Fri, 06 May 2022 18:01:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        0day robot <lkp@...el.com>, Arnd Bergmann <arnd@...db.de>,
        Theodore Ts'o <tytso@....edu>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        linux-crypto@...r.kernel.org, nathan@...nel.org
Subject: Re: [timekeeping]  3aeaac747d: PANIC:early_exception

On Fri, May 06 2022 at 12:12, Jason A. Donenfeld wrote:
> On Fri, May 06, 2022 at 09:59:13AM +0200, Thomas Gleixner wrote:
>> +/**
>> + * random_get_entropy_fallback - Returns the raw clock source value,
>> + * used by random.c for platforms with no valid random_get_entropy().
>> + */
>> +unsigned long random_get_entropy_fallback(void)
>> +{
>> +	struct tk_read_base *tkr = &tk_core.timekeeper.tkr_mono;
>> +	struct clocksource *clock = READ_ONCE(tkr->clock);
>> +
>> +	if (!timekeeping_suspended && clock)
>> +		return clock->read(clock);
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(random_get_entropy_fallback);
>
> I tried to address this already in
> <https://lore.kernel.org/lkml/20220505002910.IAcnpEOE2zR2ibERl4Lh3Y_PMmtb0Rf43lVevgztJiM@z/>,
> though yours looks better with the READ_ONCE() around clock, and I'll
> send a v8 doing it that way. I didn't realize that clock could become
> NULL again after becoming non-NULL.

This happens at early boot where clock is NULL.

> I'm not quite sure I understand the purpose of !timekeeping_suspended
> there, though. I'm not seeing the path where reading with it suspended
> negatively affects things. I'll take your word for it though.

Some clocks are not accessible during suspend.

Thanks,

        tglx