Paul Mackerras 9fb1b36ca1 powerpc: Make sure IPI handlers see data written by IPI senders ... We have been observing hangs, both of KVM guest vcpu tasks and more generally, where a process that is woken doesn't properly wake up and continue to run, but instead sticks in TASK_WAKING state. This happens because the update of rq->wake_list in ttwu_queue_remote() is not ordered with the update of ipi_message in smp_muxed_ipi_message_pass(), and the reading of rq->wake_list in scheduler_ipi() is not ordered with the reading of ipi_message in smp_ipi_demux(). Thus it is possible for the IPI receiver not to see the updated rq->wake_list and therefore conclude that there is nothing for it to do. In order to make sure that anything done before smp_send_reschedule() is ordered before anything done in the resulting call to scheduler_ipi(), this adds barriers in smp_muxed_message_pass() and smp_ipi_demux(). The barrier in smp_muxed_message_pass() is a full barrier to ensure that there is a full ordering between the smp_send_reschedule() caller and scheduler_ipi(). In smp_ipi_demux(), we use xchg() rather than xchg_local() because xchg() includes release and acquire barriers. Using xchg() rather than xchg_local() makes sense given that ipi_message is not just accessed locally. This moves the barrier between setting the message and calling the cause_ipi() function into the individual cause_ipi implementations. Most of them -- those that used outb, out_8 or similar -- already had a full barrier because out_8 etc. include a sync before the MMIO store. This adds an explicit barrier in the two remaining cases. These changes made no measurable difference to the speed of IPIs as measured using a simple ping-pong latency test across two CPUs on different cores of a POWER7 machine. The analysis of the reason why processes were not waking up properly is due to Milton Miller. Cc: stable@vger.kernel.org # v3.0+ Reported-by: Milton Miller <miltonm@bga.com> Signed-off-by: Paul Mackerras <paulus@samba.org> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>		2012-09-05 16:05:22 +10:00
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icp-hv.c	powerpc: Make sure IPI handlers see data written by IPI senders	2012-09-05 16:05:22 +10:00
icp-native.c	irqdomain: Eliminate dedicated radix lookup functions	2012-07-11 16:16:00 +01:00
ics-opal.c	powerpc/powernv: Add OPAL ICS backend	2011-09-20 16:09:59 +10:00
ics-rtas.c	powerpc/irq: Stop exporting irq_map	2011-05-04 15:02:15 +10:00
Kconfig	powerpc: Add kconfig for muxed smp ipi support	2011-05-19 15:31:05 +10:00
Makefile	powerpc/powernv: Add OPAL ICS backend	2011-09-20 16:09:59 +10:00
xics-common.c	irqdomain: Always update revmap when setting up a virq	2012-07-11 16:15:34 +01:00