Re: [Qemu-devel] [PATCH 1/1] seqlock: Fix warning reg. incompatible cast

[Paolo Bonzini]

On 08/08/2016 11:27, Paolo Bonzini wrote:
> 
> 
> On 08/08/2016 11:05, Paolo Bonzini wrote:
>>>> /home/pranith/devops/code/qemu/include/qemu/seqlock.h:62:21: warning: 
>>>> passing 'typeof (*&sl->sequence) *' (aka 'const unsigned int *') to 
>>>> parameter of type 'unsigned int *' discards qualifier
>>>> s [-Wincompatible-pointer-types-discards-qualifiers]
>>>>     return unlikely(atomic_read(&sl->sequence) != start);
>>>>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> /home/pranith/devops/code/qemu/include/qemu/atomic.h:58:25: note: expanded 
>>>> from macro 'atomic_read'
>>>>      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
>>>>                         ^~~~~
>>>> /home/pranith/devops/code/qemu/include/qemu/compiler.h:62:43: note: 
>>>> expanded from macro 'unlikely'
>>>> #define unlikely(x)   __builtin_expect(!!(x), 0)
>>>>
>>>> Signed-off-by: Pranith Kumar <address@hidden>
>> This is a compiler bug, isn't it?  Atomic loads of a const pointer
>> should be allowed.
> 
> Oh, this is &_val having a const type.  That makes more sense indeed,
> but it should be worked around in qemu/atomic.h.
> 
> Can you check if this works?

Better:

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 7e13fca..81cd33d 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -18,6 +18,26 @@
 /* Compiler barrier */
 #define barrier()   ({ asm volatile("" ::: "memory"); (void)0; })
 
+#define typeof_strip_const(expr)                                               
\
+  typeof(                                                                      
\
+    __builtin_choose_expr(                                                     
\
+      __builtin_types_compatible_p(typeof(expr), const short) ||               
\
+        __builtin_types_compatible_p(typeof(expr), short),                     
\
+      (short)1,                                                                
\
+      __builtin_choose_expr(                                                   
\
+        __builtin_types_compatible_p(typeof(expr), const unsigned short) ||    
\
+          __builtin_types_compatible_p(typeof(expr), unsigned short),          
\
+        (unsigned short)1,                                                     
\
+        __builtin_choose_expr(                                                 
\
+          __builtin_types_compatible_p(typeof(expr), const char) ||            
\
+            __builtin_types_compatible_p(typeof(expr), char),                  
\
+          (char)1,                                                             
\
+          __builtin_choose_expr(                                               
\
+            __builtin_types_compatible_p(typeof(expr), const unsigned char) || 
\
+              __builtin_types_compatible_p(typeof(expr), unsigned char),       
\
+            (unsigned char)1,                                                  
\
+            expr+0)))))
+
 #ifdef __ATOMIC_RELAXED
 /* For C11 atomic ops */
 
@@ -54,7 +74,7 @@
 #define atomic_read(ptr)                              \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof_strip_const(*ptr) _val;                    \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
     _val;                                             \
     })
@@ -80,7 +100,7 @@
 #define atomic_rcu_read(ptr)                          \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof_strip_const(*ptr) _val;                    \
     atomic_rcu_read__nocheck(ptr, &_val);             \
     _val;                                             \
     })
@@ -103,7 +123,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof_strip_const(*ptr) _val;                      \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);       \
      smp_rmb();                                         \
     _val;                                               \
@@ -120,7 +140,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof_strip_const(*ptr) _val;                      \
     __atomic_load(ptr, &_val, __ATOMIC_SEQ_CST);        \
     _val;                                               \
     })
@@ -137,7 +157,7 @@
 
 #define atomic_xchg(ptr, i)    ({                           \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));       \
-    typeof(*ptr) _new = (i), _old;                          \
+    typeof_strip_const(*ptr) _new = (i), _old;              \
     __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
     _old;                                                   \
 })
@@ -146,7 +166,7 @@
 #define atomic_cmpxchg(ptr, old, new)                                   \
     ({                                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));                   \
-    typeof(*ptr) _old = (old), _new = (new);                            \
+    typeof_strip_const(*ptr) _old = (old), _new = (new);                \
     __atomic_compare_exchange(ptr, &_old, &_new, false,                 \
                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
     _old;                                                               \


Paolo