linux-kernel - Re: [PATCH RFC] timekeeping: don't use seqcount loop in ktime_mono_to

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Message-ID: <87ttena56d.ffs@tglx>
Date: Tue, 10 Sep 2024 17:39:54 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jeff Layton <jlayton@...nel.org>, John Stultz <jstultz@...gle.com>,
 Stephen Boyd <sboyd@...nel.org>
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] timekeeping: don't use seqcount loop in
 ktime_mono_to_any on 64-bit arch

On Tue, Sep 10 2024 at 08:32, Jeff Layton wrote:
> On Tue, 2024-09-10 at 13:58 +0200, Thomas Gleixner wrote:
>> > +#if BITS_PER_LONG == 64
>> > +ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
>> > +{
>> > +	ktime_t *offset = offsets[offs];
>> > +
>> > +	return ktime_add(tmono, READ_ONCE(*offset));
>> 
>> Where is the corresponing WRITE_ONCE()?
>> 
> I'll just make it do a simple fetch without READ_ONCE.

Which will make KCSAN complain ...

So yes, READ_ONCE() is the correct thing todo, but then we want to have
the counterpart at the write sides.

Thanks,

        tglx