Re: [PATCH v2] x86/hotplug: Silence APIC only after all irq's are migrated

[Randy Dunlap]

On 8/20/20 5:42 PM, Ashok Raj wrote:
> When offlining CPUs, fixup_irqs() migrates all interrupts away from the
> outgoing CPU to an online CPU. It's always possible the device sent an
> interrupt to the previous CPU destination. Pending interrupt bit in IRR in
> LAPIC identifies such interrupts. apic_soft_disable() will not capture any
> new interrupts in IRR. This causes interrupts from device to be lost during
> CPU offline. The issue was found when explicitly setting MSI affinity to a
> CPU and immediately offlining it. It was simple to recreate with a USB
> ethernet device and doing I/O to it while the CPU is offlined. Lost
> interrupts happen even when Interrupt Remapping is enabled.
> 
> Current code does apic_soft_disable() before migrating interrupts.
> 
> native_cpu_disable()
> {
> 	...
> 	apic_soft_disable();
> 	cpu_disable_common();
> 	  --> fixup_irqs(); // Too late to capture anything in IRR.
> }
> 
> Just flipping the above call sequence seems to hit the IRR checks
> and the lost interrupt is fixed for both legacy MSI and when
> interrupt remapping is enabled.
> 
> Fixes: 60dcaad5736f ("x86/hotplug: Silence APIC and NMI when CPU is dead")
> Link: https://lore.kernel.org/lkml/875zdarr4h.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> Reported-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Tested-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
> Tested-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Reviewed-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> ---
> v2:
> - Typos and fixes suggested by Randy Dunlap

Those all look good now.  Thanks for the update.

> To: linux-kernel@xxxxxxxxxxxxxxx
> To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Sukumar Ghorai <sukumar.ghorai@xxxxxxxxx>
> Cc: Srikanth Nandamuri <srikanth.nandamuri@xxxxxxxxx>
> Cc: Evan Green <evgreen@xxxxxxxxxxxx>
> Cc: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/x86/kernel/smpboot.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)


-- 
~Randy