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

On 11.03.20 10:36, Jan Beulich wrote:
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.

This was just meant as an answer to your question regarding considering
preempt_count(). Just changing this patch here and then undoing the
change again in patch 5 is no option IMO, so I gave a hint why using
preempt_count() might be a bad idea.


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