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Merge branch '1449-atomic-fallback-safety' into 'master'

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gatomic: Reorder memory barriers in fallback atomic operations

Closes #1449

See merge request GNOME/glib!1122
This commit is contained in:
Sebastian Dröge 2019-09-24 12:50:14 +00:00
commit 4170528bd5
1 changed files with 40 additions and 4 deletions
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44
glib/gatomic.h
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@ -120,32 +120,68 @@ G_END_DECLS
#else /* defined(__ATOMIC_SEQ_CST) */
/* We want to achieve __ATOMIC_SEQ_CST semantics here. See
* https://en.cppreference.com/w/c/atomic/memory_order#Constants. For load
* operations, that means performing an *acquire*:
* > A load operation with this memory order performs the acquire operation on
* > the affected memory location: no reads or writes in the current thread can
* > be reordered before this load. All writes in other threads that release
* > the same atomic variable are visible in the current thread.
*
* “no reads or writes in the current thread can be reordered before this load”
* is implemented using a compiler barrier (a no-op `__asm__` section) to
* prevent instruction reordering. Writes in other threads are synchronised
* using `__sync_synchronize()`. It’s unclear from the GCC documentation whether
* `__sync_synchronize()` acts as a compiler barrier, hence our explicit use of
* one.
*
* For store operations, `__ATOMIC_SEQ_CST` means performing a *release*:
* > A store operation with this memory order performs the release operation:
* > no reads or writes in the current thread can be reordered after this store.
* > All writes in the current thread are visible in other threads that acquire
* > the same atomic variable (see Release-Acquire ordering below) and writes
* > that carry a dependency into the atomic variable become visible in other
* > threads that consume the same atomic (see Release-Consume ordering below).
*
* “no reads or writes in the current thread can be reordered after this store”
* is implemented using a compiler barrier to prevent instruction reordering.
* “All writes in the current thread are visible in other threads” is implemented
* using `__sync_synchronize()`; similarly for “writes that carry a dependency”.
*/
#define g_atomic_int_get(atomic) \
(G_GNUC_EXTENSION ({ \
gint gaig_result; \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
(void) (0 ? *(atomic) ^ *(atomic) : 1); \
gaig_result = (gint) *(atomic); \
__sync_synchronize (); \
(gint) *(atomic); \
__asm__ __volatile__ ("" : : : "memory"); \
gaig_result; \
}))
#define g_atomic_int_set(atomic, newval) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
(void) (0 ? *(atomic) ^ (newval) : 1); \
*(atomic) = (newval); \
__sync_synchronize (); \
__asm__ __volatile__ ("" : : : "memory"); \
*(atomic) = (newval); \
}))
#define g_atomic_pointer_get(atomic) \
(G_GNUC_EXTENSION ({ \
gpointer gapg_result; \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
gapg_result = (gpointer) *(atomic); \
__sync_synchronize (); \
(gpointer) *(atomic); \
__asm__ __volatile__ ("" : : : "memory"); \
gapg_result; \
}))
#define g_atomic_pointer_set(atomic, newval) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
(void) (0 ? (gpointer) *(atomic) : NULL); \
*(atomic) = (__typeof__ (*(atomic))) (gsize) (newval); \
__sync_synchronize (); \
__asm__ __volatile__ ("" : : : "memory"); \
*(atomic) = (__typeof__ (*(atomic))) (gsize) (newval); \
}))
#endif /* !defined(__ATOMIC_SEQ_CST) */