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powerpc: Make sure IPI handlers see data written by IPI senders

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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>
This commit is contained in:
Paul Mackerras
2012-09-04 18:33:08 +00:00
committed by Benjamin Herrenschmidt
parent 714332858b
commit 9fb1b36ca1
3 changed files with 16 additions and 3 deletions
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11
arch/powerpc/kernel/smp.c
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@@ -198,8 +198,15 @@ void smp_muxed_ipi_message_pass(int cpu, int msg)
struct cpu_messages *info = &per_cpu(ipi_message, cpu);
char *message = (char *)&info->messages;
/*
* Order previous accesses before accesses in the IPI handler.
*/
smp_mb();
message[msg] = 1;
mb();
/*
* cause_ipi functions are required to include a full barrier
* before doing whatever causes the IPI.
*/
smp_ops->cause_ipi(cpu, info->data);
}
@@ -211,7 +218,7 @@ irqreturn_t smp_ipi_demux(void)
mb(); /* order any irq clear */
do {
all = xchg_local(&info->messages, 0);
all = xchg(&info->messages, 0);
#ifdef __BIG_ENDIAN
if (all & (1 << (24 - 8 * PPC_MSG_CALL_FUNCTION)))