On Sat, Aug 25, 2012 at 03:04:23PM +1000, Paul Mackerras wrote:
> We have been observing hangs, both in KVM guest vcpus 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 calling
> 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.
>
> 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: sta...@vger.kernel.org
> Reported-by: Milton Miller <milt...@bga.com>
> Signed-off-by: Paul Mackerras <pau...@samba.org>
Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> ---
> arch/powerpc/kernel/smp.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index e292ff2..ca1040a 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -197,8 +197,15 @@ 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();
> + /*
> + * Order setting of message before IPI.
> + */
> + smp_wmb();
> smp_ops->cause_ipi(cpu, info->data);
> }
>
> @@ -210,7 +217,7 @@ 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)))
> --
> 1.7.10.rc3.219.g53414
>
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