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

[Paolo Bonzini]

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?

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 7e13fca..9c664a7 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -54,7 +54,7 @@
 #define atomic_read(ptr)                              \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof(*ptr+0) _val;                              \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
     _val;                                             \
     })
@@ -80,7 +80,7 @@
 #define atomic_rcu_read(ptr)                          \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof(*ptr+0) _val;                              \
     atomic_rcu_read__nocheck(ptr, &_val);             \
     _val;                                             \
     })
@@ -103,7 +103,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof(*ptr+0) _val;                                \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);       \
      smp_rmb();                                         \
     _val;                                               \
@@ -120,7 +120,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof(*ptr+0) _val;                                \
     __atomic_load(ptr, &_val, __ATOMIC_SEQ_CST);        \
     _val;                                               \
     })
@@ -137,7 +137,7 @@
 
 #define atomic_xchg(ptr, i)    ({                           \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));       \
-    typeof(*ptr) _new = (i), _old;                          \
+    typeof(*ptr+0) _new = (i), _old;                        \
     __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
     _old;                                                   \
 })
@@ -146,7 +146,7 @@
 #define atomic_cmpxchg(ptr, old, new)                                   \
     ({                                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));                   \
-    typeof(*ptr) _old = (old), _new = (new);                            \
+    typeof(*ptr+0) _old = (old), _new = (new);                          \
     __atomic_compare_exchange(ptr, &_old, &_new, false,                 \
                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
     _old;                                                               \