Re: tick/sched: Make jiffies update quick check more robust

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Dec 04, 2020 at 11:55:19AM +0100, Thomas Gleixner wrote:
> The quick check in tick_do_update_jiffies64() whether jiffies need to be
> updated is not really correct under all circumstances and on all
> architectures, especially not on 32bit systems.
> 
> The quick check does:
> 
>     if (now < READ_ONCE(tick_next_period))
>     	return;
> 
> and the counterpart in the update is:
> 
>     WRITE_ONCE(tick_next_period, next_update_time);
> 
> This has two problems:
> 
>   1) On weakly ordered architectures there is no guarantee that the stores
>      before the WRITE_ONCE() are visible which means that other CPUs can
>      operate on a stale jiffies value.
> 
>   2) On 32bit the store of tick_next_period which is an u64 is split into
>      two 32bit stores. If the first 32bit store advances tick_next_period
>      far out and the second 32bit store is delayed (virt, NMI ...) then
>      jiffies will become stale until the second 32bit store happens.
> 
> Address this by seperating the handling for 32bit and 64bit.
> 
> On 64bit problem #1 is addressed by replacing READ_ONCE() / WRITE_ONCE()
> with smp_load_acquire() / smp_store_release().
> 
> On 32bit problem #2 is addressed by protecting the quick check with the
> jiffies sequence counter. The load and stores can be plain because the
> sequence count mechanics provides the required barriers already.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Looks very good! Thanks!

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>