From 3603a2512f9e69dc87914ba922eb4a0812b21cd6 Mon Sep 17 00:00:00 2001 From: Don Zickus Date: Thu, 13 Oct 2011 15:14:25 -0400 Subject: [PATCH] x86, reboot: Use NMI instead of REBOOT_VECTOR to stop cpus A recent discussion started talking about the locking on the pstore fs and how it relates to the kmsg infrastructure. We noticed it was possible for userspace to r/w to the pstore fs (grabbing the locks in the process) and block the panic path from r/w to the same fs. The reason was the cpu with the lock could be doing work while the crashing cpu is panic'ing. Busting those spinlocks might cause those cpus to step on each other's data. Fine, fair enough. It was suggested it would be nice to serialize the panic path (ie stop the other cpus) and have only one cpu running. This would allow us to bust the spinlocks and not worry about another cpu stepping on the data. Of course, smp_send_stop() does this in the panic case. kmsg_dump() would have to be moved to be called after it. Easy enough. The only problem is on x86 the smp_send_stop() function calls the REBOOT_VECTOR. Any cpu with irqs disabled (which pstore and its backend ERST would do), block this IPI and thus do not stop. This makes it difficult to reliably log data to the pstore fs. The patch below switches from the REBOOT_VECTOR to NMI (and mimics what kdump does). Switching to NMI allows us to deliver the IPI when irqs are disabled, increasing the reliability of this function. However, Andi carefully noted that on some machines this approach does not work because of broken BIOSes or whatever. To help accomodate this, the next couple of patches will run a selftest and provide a knob to disable. V2: uses atomic ops to serialize the cpu that shuts everyone down V3: comment cleanup Signed-off-by: Don Zickus Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Robert Richter Cc: seiji.aguchi@hds.com Cc: vgoyal@redhat.com Cc: mjg@redhat.com Cc: tony.luck@intel.com Cc: gong.chen@intel.com Cc: satoru.moriya@hds.com Cc: avi@redhat.com Cc: Andi Kleen Link: http://lkml.kernel.org/r/1318533267-18880-2-git-send-email-dzickus@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/smp.c | 59 +++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 57 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c index 16204dc15484..e72b1754a2d7 100644 --- a/arch/x86/kernel/smp.c +++ b/arch/x86/kernel/smp.c @@ -29,6 +29,7 @@ #include #include #include +#include /* * Some notes on x86 processor bugs affecting SMP operation: * @@ -148,6 +149,60 @@ void native_send_call_func_ipi(const struct cpumask *mask) free_cpumask_var(allbutself); } +static atomic_t stopping_cpu = ATOMIC_INIT(-1); + +static int smp_stop_nmi_callback(unsigned int val, struct pt_regs *regs) +{ + /* We are registered on stopping cpu too, avoid spurious NMI */ + if (raw_smp_processor_id() == atomic_read(&stopping_cpu)) + return NMI_HANDLED; + + stop_this_cpu(NULL); + + return NMI_HANDLED; +} + +static void native_nmi_stop_other_cpus(int wait) +{ + unsigned long flags; + unsigned long timeout; + + if (reboot_force) + return; + + /* + * Use an own vector here because smp_call_function + * does lots of things not suitable in a panic situation. + */ + if (num_online_cpus() > 1) { + /* did someone beat us here? */ + if (atomic_cmpxchg(&stopping_cpu, -1, safe_smp_processor_id() != -1)) + return; + + if (register_nmi_handler(NMI_LOCAL, smp_stop_nmi_callback, + NMI_FLAG_FIRST, "smp_stop")) + /* Note: we ignore failures here */ + return; + + /* sync above data before sending NMI */ + wmb(); + + apic->send_IPI_allbutself(NMI_VECTOR); + + /* + * Don't wait longer than a second if the caller + * didn't ask us to wait. + */ + timeout = USEC_PER_SEC; + while (num_online_cpus() > 1 && (wait || timeout--)) + udelay(1); + } + + local_irq_save(flags); + disable_local_APIC(); + local_irq_restore(flags); +} + /* * this function calls the 'stop' function on all other CPUs in the system. */ @@ -160,7 +215,7 @@ asmlinkage void smp_reboot_interrupt(void) irq_exit(); } -static void native_stop_other_cpus(int wait) +static void native_irq_stop_other_cpus(int wait) { unsigned long flags; unsigned long timeout; @@ -230,7 +285,7 @@ struct smp_ops smp_ops = { .smp_prepare_cpus = native_smp_prepare_cpus, .smp_cpus_done = native_smp_cpus_done, - .stop_other_cpus = native_stop_other_cpus, + .stop_other_cpus = native_nmi_stop_other_cpus, .smp_send_reschedule = native_smp_send_reschedule, .cpu_up = native_cpu_up, -- 2.39.5