Re: [Xen-devel] [PATCH v4 3/6] xen: add process_pending_softirqs_norcu() for keyhandlers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.03.2020 10:27, Jürgen Groß wrote:
> On 11.03.20 10:25, Jan Beulich wrote:
>> On 11.03.2020 07:07, Jürgen Groß wrote:
>>> On 10.03.20 18:02, Jan Beulich wrote:
>>>> On 10.03.2020 08:28, Juergen Gross wrote:
>>>>> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
>>>>> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
>>>>> @@ -587,7 +587,7 @@ static void amd_dump_p2m_table_level(struct 
>>>>> page_info* pg, int level,
>>>>>            struct amd_iommu_pte *pde = &table_vaddr[index];
>>>>>    
>>>>>            if ( !(index % 2) )
>>>>> -            process_pending_softirqs();
>>>>> +            process_pending_softirqs_norcu();
>>>>
>>>> At the example of this - the property of holding an RCU lock is
>>>> entirely invisible here, as it's the generic
>>>> iommu_dump_p2m_table() which acquires it. This suggest to me that
>>>> going forward breaking this is going to be very likely. Couldn't
>>>> process_pending_softirqs() exclude RCU handling when finding
>>>> preempt_count() to return non-zero?
>>>
>>> This can be done, but then the non-debug build would require to have
>>> non-empty rcu lock functions.
>>
>> I guess I don't understand - I see only one version of them:
>>
>> #define rcu_read_lock(x)       ({ ((void)(x)); preempt_disable(); })
>> #define rcu_read_unlock(x)     ({ ((void)(x)); preempt_enable(); })
>>
>> Same for the preempt count adjustment operations.
> 
> See patch 5.

Which I haven't looked at yet, and which I also shouldn't need to
look at to understand the patch here. If this is a preparatory
change rather than some form of fix or improvement, then the
description should say so.

Jan

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