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....@nanos.tec.linutronix.de/
Reported-by: Evan Green <evgr...@chromium.org>
Tested-by: Mathias Nyman <mathias.ny...@linux.intel.com>
Tested-by: Evan Green <evgr...@chromium.org>
Reviewed-by: Evan Green <evgr...@chromium.org>
Signed-off-by: Ashok Raj <ashok....@intel.com>
---
v3:
Fix commit message and comments per Thomas.
https://lore.kernel.org/lkml/87mu2iw86q....@nanos.tec.linutronix.de/
v2:
- Typos and fixes suggested by Randy Dunlap
To: linux-kernel@vger.kernel.org
To: Thomas Gleixner <t...@linutronix.de>
Cc: Sukumar Ghorai <sukumar.gho...@intel.com>
Cc: Srikanth Nandamuri <srikanth.nandam...@intel.com>
Cc: Evan Green <evgr...@chromium.org>
Cc: Mathias Nyman <mathias.ny...@linux.intel.com>
Cc: Bjorn Helgaas <bhelg...@google.com>
Cc: x...@kernel.org
Cc: sta...@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;
}
--
2.7.4
