From 9fb1b36ca1234e64a5d1cc573175303395e3354d Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Tue, 4 Sep 2012 18:33:08 +0000 Subject: [PATCH] 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 Signed-off-by: Paul Mackerras Signed-off-by: Benjamin Herrenschmidt --- arch/powerpc/kernel/dbell.c | 2 ++ arch/powerpc/kernel/smp.c | 11 +++++++++-- arch/powerpc/sysdev/xics/icp-hv.c | 6 +++++- 3 files changed, 16 insertions(+), 3 deletions(-) diff --git a/arch/powerpc/kernel/dbell.c b/arch/powerpc/kernel/dbell.c index 5b25c8060fd6..a892680668d8 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); } diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 0321007086f7..8d4214afc21d 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -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))) diff --git a/arch/powerpc/sysdev/xics/icp-hv.c b/arch/powerpc/sysdev/xics/icp-hv.c index 14469cf9df68..df0fc5821469 100644 --- a/arch/powerpc/sysdev/xics/icp-hv.c +++ b/arch/powerpc/sysdev/xics/icp-hv.c @@ -65,7 +65,11 @@ static inline void icp_hv_set_xirr(unsigned int value) static inline void icp_hv_set_qirr(int n_cpu , u8 value) { int hw_cpu = get_hard_smp_processor_id(n_cpu); - long rc = plpar_hcall_norets(H_IPI, hw_cpu, value); + long rc; + + /* Make sure all previous accesses are ordered before IPI sending */ + mb(); + rc = plpar_hcall_norets(H_IPI, hw_cpu, value); if (rc != H_SUCCESS) { pr_err("%s: bad return code qirr cpu=%d hw_cpu=%d mfrr=0x%x " "returned %ld\n", __func__, n_cpu, hw_cpu, value, rc); -- 2.20.1