Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

[Thomas Gleixner]

On Mon, Sep 16 2024 at 12:12, Thomas Gleixner wrote:
> On Sat, Sep 14 2024 at 13:07, Jeff Layton wrote:
>> +	do {
>> +		seq = read_seqcount_begin(&tk_core.seq);
>> +
>> +		ts->tv_sec = tk->xtime_sec;
>> +		mono = tk->tkr_mono.base;
>> +		nsecs = timekeeping_get_ns(&tk->tkr_mono);
>> +		offset = *offsets[TK_OFFS_REAL];
>> +	} while (read_seqcount_retry(&tk_core.seq, seq));
>> +
>> +	mono = ktime_add_ns(mono, nsecs);
>> +
>> +	if (atomic64_try_cmpxchg(&mg_floor, &old, mono)) {
>> +		ts->tv_nsec = 0;
>> +		timespec64_add_ns(ts, nsecs);
>> +	} else {
>> +		/*
>> +		 * Something has changed mg_floor since "old" was
>> +		 * fetched. "old" has now been updated with the
>> +		 * current value of mg_floor, so use that to return
>> +		 * the current coarse floor value.
>
> 'Something has changed' is a truly understandable technical
> explanation.

     old = mg_floor
                                mono = T1;
                                mg_floor = mono
preemption

     do {
        mono = T2;
     }

     cmpxchg fails and the function returns a value based on T1

No?

Thanks,

        tglx