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

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

On 06/06/17 18:06, Andre Przywara wrote:
> On 30/05/17 11:47, 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.
>
> The patch you sent yesterday may be a bit over the top for this.

Patch which I only sent internally to get feedback. ;)

>
> What about this one (in xen/arch/arm/vgic-v[23].c):
>
> static int vgic_v2_distr_mmio_write(struct vcpu *v, ...
> {
> -    uint32_t *ipriorityr;
> +    uint32_t *ipriorityr, priority;
> ....
>      vgic_lock_rank(v, rank, flags);
>      ipriorityr = &rank->ipriorityr[REG_RANK_INDEX(8,
>                                             gicd_reg - GICD_IPRIORITYR,
>                                             DABT_WORD)];
> -    vgic_reg32_update(ipriorityr, r, info);
> +    priority = ACCESS_ONCE(*ipriorityr);
> +    vgic_reg32_update(&priority, r, info);
> +    ACCESS_ONCE(*ipriorityr) = priority;
>      vgic_unlock_rank(v, rank, flags);
> ....

What you do is very similar to my patch but open-code it. It might be 
possible to have other place which could take advantage of the a generic 
solution, so I am not convinced this would be the right way.

>
> Concurrent writes are protected by the rank lock, and reads are
> guaranteed to be atomic due to ACCESS_ONCE and the architectural
> guarantee of an aligned uint32_t access being atomic.
> We can't do anything about the benign race when the priority gets
> changed while we read it, but the above should make sure that we either
> read the old or the new value and nothing in between.
> The other read accesses (in vgic_v[23]_distr_mmio_read() and in vgic.c
> here in this patch) get protected by an ACCESS_ONCE().
>
> Does that make sense?

I was trying to address the suggestion of Stefano to drop the rank lock 
in some place and also make the solution generic.

I failed to address properly the lock drop. And have some idea how to do it.

Cheers,

-- 
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel