Re: [PATCH v2 1/1] sparc64: Fix comparator problem with timer interrupts

[Thomas Gleixner <tglx@kernel.org>]

On Mon, May 18 2026 at 19:24, Tony Rodriguez wrote:
> On SPARC64 the check:
>
>     return ((long)(new_tick - (orig_tick + adj))) > 0L;
>
> Is safe only if retries make forward progress. The comparator can
> take effect with a latency, so the moment when counter == comparator
> may be missed, which can cause delays or hangs on some SPARC64 systems.
>
> For clarity:
>     exp = orig_tick + adj   /* expected comparator value */
>
> The current check requires new_tick to be strictly greater than exp;
> equality (new_tick == exp) is treated as not yet passed and the caller
> will retry.

That's confusing at best. You really want to explain how the ordering is
similar to what I described in the analysis:

        exp = read_cnt() + delta_ticks;
        write_cmp(exp);
        return (read_cnt() - exp) > 0;

If the counter advanced past the expected expiry time, after writing it,
then the caller will retry, as the calling code does:

     return tick.add_compare(delta_ticks) ? -ETIME : 0;

But it won't do so when the counter is equal, which is causing the
problem.

> By contrast, using:
>
>     return ((long)(new_tick - (orig_tick + adj))) >= 0L;
>
> causes the caller to stop retrying and assume the timer is scheduled;
> both equality and greater-than are accepted (new_tick == exp or
> new_tick > exp).

It's the other way round. When counter >= expiry time, then the write is
considered failed. If the counter has not yet reached expiry time,
i.e. it is smaller, then it assumes the timer is scheduled.

> Signed-off-by: Tony Rodriguez <unixpro1970@gmail.com>

It would be nice to have a link to the original thread in the change log
itself as that gives people quick access when they are wondering about
this a year down the road.

Thanks,

        tglx