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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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