Am 07.11.2010 11:03, Philippe Gerum wrote: > On Sun, 2010-11-07 at 09:31 +0100, Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>>>> Anyway, after some thoughts, I think we are going to try and make the >>>>> current situation work instead of going back to the old way. >>>>> >>>>> You can find the patch which attempts to do so here: >>>>> http://sisyphus.hd.free.fr/~gilles/sched_status.txt >>>> Ack. At last, this addresses the real issues without asking for >>>> regression funkiness: fix the lack of barrier before testing XNSCHED in >>> >>> Check the kernel, we actually need it on both sides. Wherever the final >>> barriers will be, we should leave a comment behind why they are there. >>> Could be picked up from kernel/smp.c. >> >> We have it on both sides: the non-local flags are modified while holding >> the nklock. Unlocking the nklock implies a barrier. > > I think we may have an issue with this kind of construct: > > xnlock_get_irq*(&nklock) > xnpod_resume/suspend/whatever_thread() > xnlock_get_irq*(&nklock) > ... > xnlock_put_irq*(&nklock) > xnpod_schedule() > xnlock_get_irq*(&nklock) > send_ipi > =====> xnpod_schedule_handler on dest CPU > xnlock_put_irq*(&nklock) > xnlock_put_irq*(&nklock) > > The issue would be triggered by the use of recursive locking. In that > case, the source CPU would only sync its cache when the lock is actually > dropped by the outer xnlock_put_irq* call and the inner > xnlock_get/put_irq* would not act as barriers, so the remote > rescheduling handler won't always see the XNSCHED update done remotely, > and may lead to a no-op. So we need a barrier before sending the IPI in > __xnpod_test_resched().
That's what I said. And we need it on the reader side as an rmb(). Jan
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