linux/arch/powerpc/sysdev/xics
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
..
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
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
xics-common.c irqdomain: Always update revmap when setting up a virq 2012-07-11 16:15:34 +01:00