[xen stable-4.16] x86/irq: do not release irq until all cleanup is done

[patchbot@xxxxxxx]

commit b6b3dc8d887b4d78b802c236d93dc792c8355074
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Dec 8 10:10:00 2022 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Dec 8 10:10:00 2022 +0100

    x86/irq: do not release irq until all cleanup is done
    
    Current code in _clear_irq_vector() will mark the irq as unused before
    doing the cleanup required when move_in_progress is true.
    
    This can lead to races in create_irq() if the function picks an irq
    desc that's been marked as unused but has move_in_progress set, as the
    call to assign_irq_vector() in that function can then fail with
    -EAGAIN.
    
    Prevent that by only marking irq descs as unused when all the cleanup
    has been done.  While there also use write_atomic() when setting
    IRQ_UNUSED in _clear_irq_vector() and add a barrier in order to
    prevent the setting of IRQ_UNUSED getting reordered by the compiler.
    
    The check for move_in_progress cannot be removed from
    _assign_irq_vector(), as other users (io_apic_set_pci_routing() and
    ioapic_guest_write()) can still pass active irq descs to
    assign_irq_vector().
    
    Note the trace point is not moved and is now set before the irq is
    marked as unused.  This is done so that the CPU mask provided in the
    trace point is the one belonging to the current vector, not the old
    one.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: e267d11969a40f0aec33dbf966f5a6490b205f43
    master date: 2022-12-02 10:32:21 +0100
---
 xen/arch/x86/irq.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 47b86af5dc..80ce9d624f 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -222,27 +222,28 @@ static void _clear_irq_vector(struct irq_desc *desc)
         clear_bit(vector, desc->arch.used_vectors);
     }
 
-    desc->arch.used = IRQ_UNUSED;
-
     trace_irq_mask(TRC_HW_IRQ_CLEAR_VECTOR, irq, vector, tmp_mask);
 
-    if ( likely(!desc->arch.move_in_progress) )
-        return;
+    if ( unlikely(desc->arch.move_in_progress) )
+    {
+        /* If we were in motion, also clear desc->arch.old_vector */
+        old_vector = desc->arch.old_vector;
+        cpumask_and(tmp_mask, desc->arch.old_cpu_mask, &cpu_online_map);
 
-    /* If we were in motion, also clear desc->arch.old_vector */
-    old_vector = desc->arch.old_vector;
-    cpumask_and(tmp_mask, desc->arch.old_cpu_mask, &cpu_online_map);
+        for_each_cpu(cpu, tmp_mask)
+        {
+            ASSERT(per_cpu(vector_irq, cpu)[old_vector] == irq);
+            TRACE_3D(TRC_HW_IRQ_MOVE_FINISH, irq, old_vector, cpu);
+            per_cpu(vector_irq, cpu)[old_vector] = ~irq;
+        }
 
-    for_each_cpu(cpu, tmp_mask)
-    {
-        ASSERT(per_cpu(vector_irq, cpu)[old_vector] == irq);
-        TRACE_3D(TRC_HW_IRQ_MOVE_FINISH, irq, old_vector, cpu);
-        per_cpu(vector_irq, cpu)[old_vector] = ~irq;
-    }
+        release_old_vec(desc);
 
-    release_old_vec(desc);
+        desc->arch.move_in_progress = 0;
+    }
 
-    desc->arch.move_in_progress = 0;
+    smp_wmb();
+    write_atomic(&desc->arch.used, IRQ_UNUSED);
 }
 
 void __init clear_irq_vector(int irq)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.16