[PATCH for-4.21 v3 1/2] x86/apic: Avoid infinite loop in io_apic_level_ack_pending()

[Jason Andryuk <jason.andryuk@xxxxxxx>]

io_apic_level_ack_pending() will end up in an infinite loop if
entry->pin == -1.  entry does not change, so it will keep reading -1.

Convert to a proper for loop so that continue works.  Add a new helper,
next_entry(), to handle advancing to the next irq_pin_list entry.

Noticed during code inspection.  The infinite loop was not observed.

Fixes: f821102450a1 ("x86: IRQ Migration logic enhancement.")
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
v3:
const on next_entry() parameter
Remove spaces inside for loop braces
Remove inner if (!entry) check
Expand commit message to state noticed during code inspection

v2:
continue (not break) for pin == -1.

I added the next_entry() helper since putting the expression in the for
loop is a little cluttered.  The helper can also be re-used for other
instances within the file.
---
 xen/arch/x86/io_apic.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index c384f10c1b..c35d611ecf 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -1586,20 +1586,24 @@ static int __init cf_check setup_ioapic_ack(const char 
*s)
 }
 custom_param("ioapic_ack", setup_ioapic_ack);
 
+static struct irq_pin_list *next_entry(const struct irq_pin_list *entry)
+{
+    if ( !entry->next )
+        return NULL;
+
+    return irq_2_pin + entry->next;
+}
+
 static bool io_apic_level_ack_pending(unsigned int irq)
 {
     struct irq_pin_list *entry;
     unsigned long flags;
 
     spin_lock_irqsave(&ioapic_lock, flags);
-    entry = &irq_2_pin[irq];
-    for (;;) {
+    for (entry = &irq_2_pin[irq]; entry; entry = next_entry(entry)) {
         unsigned int reg;
         int pin;
 
-        if (!entry)
-            break;
-
         pin = entry->pin;
         if (pin == -1)
             continue;
@@ -1609,9 +1613,6 @@ static bool io_apic_level_ack_pending(unsigned int irq)
             spin_unlock_irqrestore(&ioapic_lock, flags);
             return 1;
         }
-        if (!entry->next)
-            break;
-        entry = irq_2_pin + entry->next;
     }
     spin_unlock_irqrestore(&ioapic_lock, flags);
 
-- 
2.51.0