@@ -326,9 +326,19 @@ and memory barriers, and the equivalents in QEMU:
use a boxed atomic_t type; atomic operations in QEMU are polymorphic
and use normal C types.
-- atomic_read and atomic_set in Linux give no guarantee at all;
- atomic_read and atomic_set in QEMU include a compiler barrier
- (similar to the ACCESS_ONCE macro in Linux).
+- Originally, atomic_read and atomic_set in Linux gave no guarantee
+ at all. Recently they have been updated to implement volatile
+ semantics via ACCESS_ONCE (or the more recent READ/WRITE_ONCE).
+
+ QEMU's atomic_read/set implement, if the compiler supports it, C11
+ atomic relaxed semantics, and volatile semantics otherwise.
+ Both semantics prevent the compiler from doing certain transformations;
+ the difference is that atomic accesses are guaranteed to be atomic,
+ while volatile accesses aren't. Thus, in the volatile case we just cross
+ our fingers hoping that the compiler will generate atomic accesses,
+ since we assume the variables passed are machine-word sized and
+ properly aligned.
+ No barriers are implied by atomic_read/set in either Linux or QEMU.
- most atomic read-modify-write operations in Linux return void;
in QEMU, all of them return the old value of the variable.
Recently Linux did a mass conversion of its atomic_read/set calls so that they at least are READ/WRITE_ONCE. See Linux's commit 62e8a325 ("atomic, arch: Audit atomic_{read,set}()"). It seems though that their documentation hasn't been updated to reflect this. The appended updates our documentation to reflect the change, which means there is effectively no difference between our atomic_read/set and the current Linux implementation. While at it, fix the statement that a barrier is implied by atomic_read/set, which is incorrect. Volatile/atomic semantics prevent transformations pertaining the variable they apply to; this, however, has no effect on surrounding statements like barriers do. For more details on this, see: https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html Signed-off-by: Emilio G. Cota <cota@braap.org> --- docs/atomics.txt | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-)