There is a race when taking a CPU offline. Current code looks like this:
native_cpu_disable() { ... apic_soft_disable(); /* * Any existing set bits for pending interrupt to * this CPU are preserved and will be sent via IPI * to another CPU by fixup_irqs(). */ cpu_disable_common(); { .... /* * Race window happens here. Once local APIC has been * disabled any new interrupts from the device to * the old CPU are lost */ fixup_irqs(); // Too late to capture anything in IRR. ... } }
The fix is to disable the APIC *after* cpu_disable_common().
Testing was done with a USB NIC that provided a source of frequent interrupts. A script migrated interrupts to a specific CPU and then took that CPU offline.
Fixes: 60dcaad5736f ("x86/hotplug: Silence APIC and NMI when CPU is dead") Link: https://lore.kernel.org/lkml/875zdarr4h.fsf@nanos.tec.linutronix.de/ Reported-by: Evan Green evgreen@chromium.org Tested-by: Mathias Nyman mathias.nyman@linux.intel.com Tested-by: Evan Green evgreen@chromium.org Reviewed-by: Evan Green evgreen@chromium.org Signed-off-by: Ashok Raj ashok.raj@intel.com --- v3: Fix commit message and comments per Thomas. https://lore.kernel.org/lkml/87mu2iw86q.fsf@nanos.tec.linutronix.de/
v2: - Typos and fixes suggested by Randy Dunlap
To: linux-kernel@vger.kernel.org To: Thomas Gleixner tglx@linutronix.de Cc: Sukumar Ghorai sukumar.ghorai@intel.com Cc: Srikanth Nandamuri srikanth.nandamuri@intel.com Cc: Evan Green evgreen@chromium.org Cc: Mathias Nyman mathias.nyman@linux.intel.com Cc: Bjorn Helgaas bhelgaas@google.com Cc: x86@kernel.org Cc: stable@vger.kernel.org --- arch/x86/kernel/smpboot.c | 26 ++++++++++++++++++++------ 1 file changed, 20 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 27aa04a95702..f5ef689dd62a 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1594,14 +1594,28 @@ int native_cpu_disable(void) if (ret) return ret;
- /* - * Disable the local APIC. Otherwise IPI broadcasts will reach - * it. It still responds normally to INIT, NMI, SMI, and SIPI - * messages. - */ - apic_soft_disable(); cpu_disable_common();
+ /* + * Disable the local APIC. Otherwise IPI broadcasts will reach + * it. It still responds normally to INIT, NMI, SMI, and SIPI + * messages. + * + * Disabling the APIC must happen after cpu_disable_common() + * which invokes fixup_irqs(). + * + * Disabling the APIC preserves already set bits in IRR, but + * an interrupt arriving after disabling the local APIC does not + * set the corresponding IRR bit. + * + * fixup_irqs() scans IRR for set bits so it can raise a not + * yet handled interrupt on the new destination CPU via an IPI + * but obviously it can't do so for IRR bits which are not set. + * IOW, interrupts arriving after disabling the local APIC will + * be lost. + */ + apic_soft_disable(); + return 0; }