linux-kernel - Re: [PATCH v2 16/25] timekeeping: Rework do_settimeofday64() to use shadow

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Message-ID: <CANDhNCoN4Xug6XKSF+-cuXxRUiLhJPrBjiCmFmsTxxknvwN7nQ@mail.gmail.com>
Date: Thu, 24 Oct 2024 14:54:40 -0700
From: John Stultz <jstultz@...gle.com>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	Miroslav Lichvar <mlichvar@...hat.com>, Richard Cochran <richardcochran@...il.com>, 
	Christopher S Hall <christopher.s.hall@...el.com>
Subject: Re: [PATCH v2 16/25] timekeeping: Rework do_settimeofday64() to use shadow_timekeeper

On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
<anna-maria@...utronix.de> wrote:
>
> From: Anna-Maria Behnsen <anna-maria@...utronix.de>
>
> Updates of the timekeeper can be done by operating on the shadow timekeeper
> and afterwards copying the result into the real timekeeper. This has the
> advantage, that the sequence count write protected region is kept as small
> as possible.
>
> Convert do_settimeofday64() to use this scheme.
>
> That allows to use a scoped_guard() for locking the timekeeper lock as the
> usage of the shadow timekeeper allows a rollback in the error case instead
> of the full timekeeper update of the original code.
>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>  kernel/time/timekeeping.c | 42 ++++++++++++++++--------------------------
>  1 file changed, 16 insertions(+), 26 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 41d88f645868..cc01ad53d96d 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1479,45 +1479,35 @@ EXPORT_SYMBOL_GPL(timekeeping_clocksource_has_base);
>   */
>  int do_settimeofday64(const struct timespec64 *ts)
>  {
> -       struct timekeeper *tk = &tk_core.timekeeper;
>         struct timespec64 ts_delta, xt;
> -       unsigned long flags;
> -       int ret = 0;
>
>         if (!timespec64_valid_settod(ts))
>                 return -EINVAL;
>
> -       raw_spin_lock_irqsave(&tk_core.lock, flags);
> -       write_seqcount_begin(&tk_core.seq);
> -
> -       timekeeping_forward_now(tk);
> -
> -       xt = tk_xtime(tk);
> -       ts_delta = timespec64_sub(*ts, xt);
> +       scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
> +               struct timekeeper *tk = &tk_core.shadow_timekeeper;

nit: maybe shadow_tk instead, so it is additionally clear in the
following logic which is being modified.

Otherwise,
Acked-by: John Stultz <jstultz@...gle.com>