diff options
| -rw-r--r-- | docs/atomics.txt | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/docs/atomics.txt b/docs/atomics.txt index bba771ecd6..67a27ad70a 100644 --- a/docs/atomics.txt +++ b/docs/atomics.txt @@ -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 READ_ONCE/WRITE_ONCE macros in Linux). +- Originally, atomic_read and atomic_set in Linux gave no guarantee + at all. Linux 4.1 updated them 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. |