From 241ee90a69ede9cf9255df1a18036210beeb8adf Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Tue, 4 Sep 2012 18:33:08 +0000 Subject: powerpc: Make sure IPI handlers see data written by IPI senders commit 9fb1b36ca1234e64a5d1cc573175303395e3354d upstream. 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. Reported-by: Milton Miller Signed-off-by: Paul Mackerras Signed-off-by: Benjamin Herrenschmidt Signed-off-by: Ben Hutchings --- arch/powerpc/kernel/dbell.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/powerpc/kernel/dbell.c') diff --git a/arch/powerpc/kernel/dbell.c b/arch/powerpc/kernel/dbell.c index 2cc451aaaca7..6856062666d6 100644 --- a/arch/powerpc/kernel/dbell.c +++ b/arch/powerpc/kernel/dbell.c @@ -28,6 +28,8 @@ void doorbell_setup_this_cpu(void) void doorbell_cause_ipi(int cpu, unsigned long data) { + /* Order previous accesses vs. msgsnd, which is treated as a store */ + mb(); ppc_msgsnd(PPC_DBELL, 0, data); } -- cgit v1.2.3