On Tue, Sep 18, 2018 at 03:23:08PM +0200, Thomas Gleixner wrote: > On Tue, 18 Sep 2018, Peter Zijlstra wrote: > > On Tue, Sep 18, 2018 at 12:41:57PM +0200, Thomas Gleixner wrote: > > > I still have one of the machines which is affected by this. > > > > Are we sure this isn't a load vs rdtsc reorder? Because if I look at the > > current code: > > The load order of last vs. rdtsc does not matter at all. > > CPU0 CPU1 > > .... > now0 = rdtsc_ordered(); > ... > tk->cycle_last = now0; > > gtod->seq++; > gtod->cycle_last = tk->cycle_last; > ... > gtod->seq++; > seq_begin(gtod->seq); > now1 = rdtsc_ordered(); > > So if the TSC on CPU1 is slightly behind the TSC on CPU0 then now1 can be > smaller than cycle_last. The TSC sync stuff does not catch the small delta > for unknown raisins. I'll go and find that machine and test that again. Yeah, somehow I forgot about rseq.. maybe I should go sleep or something. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
