Re: [PATCH v10 01/32] ARM: vGIC: avoid rank lock when reading priority

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 30/05/17 22:39, Stefano Stabellini wrote:
> On Tue, 30 May 2017, Julien Grall wrote:
>> Hi Andre,
>>
>> On 26/05/17 18:35, Andre Przywara wrote:
>>> When reading the priority value of a virtual interrupt, we were taking
>>> the respective rank lock so far.
>>> However for forwarded interrupts (Dom0 only so far) this may lead to a
>>> deadlock with the following call chain:
>>> - MMIO access to change the IRQ affinity, calling the ITARGETSR handler
>>> - this handler takes the appropriate rank lock and calls
>>> vgic_store_itargetsr()
>>> - vgic_store_itargetsr() will eventually call vgic_migrate_irq()
>>> - if this IRQ is already in-flight, it will remove it from the old
>>>   VCPU and inject it into the new one, by calling vgic_vcpu_inject_irq()
>>> - vgic_vcpu_inject_irq will call vgic_get_virq_priority()
>>> - vgic_get_virq_priority() tries to take the rank lock - again!
>>> It seems like this code path has never been exercised before.
>>>
>>> Fix this by avoiding taking the lock in vgic_get_virq_priority() (like we
>>> do in vgic_get_target_vcpu()).
>>> Actually we are just reading one byte, and priority changes while
>>> interrupts are handled are a benign race that can happen on real hardware
>>> too. So it looks safe to just use read_atomic() instead.
>>>
>>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>>> ---
>>>  xen/arch/arm/vgic.c | 8 +-------
>>>  1 file changed, 1 insertion(+), 7 deletions(-)
>>>
>>> diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
>>> index 83569b0..54b2aad 100644
>>> --- a/xen/arch/arm/vgic.c
>>> +++ b/xen/arch/arm/vgic.c
>>> @@ -227,14 +227,8 @@ struct vcpu *vgic_get_target_vcpu(struct vcpu *v,
>>> unsigned int virq)
>>>  static int vgic_get_virq_priority(struct vcpu *v, unsigned int virq)
>>>  {
>>>      struct vgic_irq_rank *rank = vgic_rank_irq(v, virq);
>>> -    unsigned long flags;
>>> -    int priority;
>>> -
>>> -    vgic_lock_rank(v, rank, flags);
>>> -    priority = rank->priority[virq & INTERRUPT_RANK_MASK];
>>> -    vgic_unlock_rank(v, rank, flags);
>>>
>>> -    return priority;
>>> +    return read_atomic(&rank->priority[virq & INTERRUPT_RANK_MASK]);
>>
>> The write in rank->priority will not be atomic (see vgic_reg_update
>> implementation): the register is first masked, the the priority set.
>>
>> So you may end up to read 0 (which is the higher priority) by mistake.
>>
>> We should probably think to make vgic_reg_* helper atomic.
>
> Right! That's why I wrote
> alpine.DEB.2.10.1705231135130.18759@sstabellini-ThinkPad-X260

It was not obvious from this e-mail why you wanted this. It looked more 
a request for improvement rather than a potential bug. Hence my 
suggestion to delay it.

Cheers,

-- 
Julien Grall

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