[PATCH v2 1/8] evtchn: avoid race in get_xen_consumer()

[Jan Beulich <jbeulich@suse.com>]

There's no global lock around the updating of this global piece of data.
Make use of cmpxchgptr() to avoid two entities racing with their
updates.

While touching the functionality, mark xen_consumers[] read-mostly (or
else the if() condition could use the result of cmpxchgptr(), writing to
the slot unconditionally).

The use of cmpxchgptr() here points out (by way of clang warning about
it) that its original use of const was slightly wrong. Adjust the
placement, or else undefined behavior of const qualifying a function
type will result.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v2: Use (and hence generalize) cmpxchgptr(). Add comment. Expand /
    adjust description.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -57,7 +57,8 @@
  * with a pointer, we stash them dynamically in a small lookup array which
  * can be indexed by a small integer.
  */
-static xen_event_channel_notification_t xen_consumers[NR_XEN_CONSUMERS];
+static xen_event_channel_notification_t __read_mostly
+    xen_consumers[NR_XEN_CONSUMERS];
 
 /* Default notification action: wake up from wait_on_xen_event_channel(). */
 static void default_xen_notification_fn(struct vcpu *v, unsigned int port)
@@ -80,8 +81,9 @@ static uint8_t get_xen_consumer(xen_even
 
     for ( i = 0; i < ARRAY_SIZE(xen_consumers); i++ )
     {
+        /* Use cmpxchgptr() in lieu of a global lock. */
         if ( xen_consumers[i] == NULL )
-            xen_consumers[i] = fn;
+            cmpxchgptr(&xen_consumers[i], NULL, fn);
         if ( xen_consumers[i] == fn )
             break;
     }
--- a/xen/include/asm-x86/system.h
+++ b/xen/include/asm-x86/system.h
@@ -148,13 +148,6 @@ static always_inline unsigned long cmpxc
     return prev;
 }
 
-#define cmpxchgptr(ptr,o,n) ({                                          \
-    const __typeof__(**(ptr)) *__o = (o);                               \
-    __typeof__(**(ptr)) *__n = (n);                                     \
-    ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)__o,            \
-                                   (unsigned long)__n,sizeof(*(ptr)))); \
-})
-
 /*
  * Undefined symbol to cause link failure if a wrong size is used with
  * arch_fetch_and_add().
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -178,6 +178,17 @@ unsigned long long parse_size_and_unit(c
 
 uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c);
 
+/*
+ * A slightly more typesafe variant of cmpxchg() when the entities dealt with
+ * are pointers.
+ */
+#define cmpxchgptr(ptr, o, n) ({                                        \
+    __typeof__(**(ptr)) *const o_ = (o);                                \
+    __typeof__(**(ptr)) *n_ = (n);                                      \
+    ((__typeof__(*(ptr)))__cmpxchg(ptr, (unsigned long)o_,              \
+                                   (unsigned long)n_, sizeof(*(ptr)))); \
+})
+
 #define TAINT_SYNC_CONSOLE              (1u << 0)
 #define TAINT_MACHINE_CHECK             (1u << 1)
 #define TAINT_ERROR_INJECT              (1u << 2)