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Re: [Xen-devel] [PATCH 1/7] Port WARN_ON_ONCE() from Linux



Hi,

On 02/09/2018 10:29 AM, Roger Pau Monné wrote:
On Thu, Feb 08, 2018 at 08:10:49PM -0700, Sameer Goel wrote:
diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 1d9771340c..697212a061 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -11,6 +11,19 @@
  #define BUG_ON(p)  do { if (unlikely(p)) BUG();  } while (0)
  #define WARN_ON(p) do { if (unlikely(p)) WARN(); } while (0)
+#define WARN_ON_ONCE(p) \
+({                                                  \
+    static bool __section(".data.unlikely") __warned; \
+    int __ret_warn_once = !!(p);                    \
        ^ bool

+                                                    \
+    if ( unlikely(__ret_warn_once && !__warned) )     \
+    {                                               \
+        __warned = true;                            \
+        WARN();                                     \
+    }                                               \
+    unlikely(__ret_warn_once);                      \

Does this macro really need to return something? It seems weird to me
to allow usages like: if ( WARN_ON_ONCE...

This construct is used in Linux (included in the driver ported):

if (WARN_ON_ONCE(fwspec->iommu_priv)) {
     master = fwspec->iommu_priv;
     smmu = master->smmu;
} else {
....
}

IHMO the makes the code nicer to read over:

WARN_ON_ONCE(...)
if ( ... ) {
} else {
}

Cheers,

--
Julien Grall

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