From 56ebe02203f033a8399f7f6ea6972225ed87101c Mon Sep 17 00:00:00 2001 From: "Emilio G. Cota" Date: Tue, 24 May 2016 16:06:12 -0400 Subject: [PATCH] docs/atomics: update atomic_read/set comparison with Linux 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 Message-Id: <1464120374-8950-2-git-send-email-cota@braap.org> Signed-off-by: Paolo Bonzini --- docs/atomics.txt | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/docs/atomics.txt b/docs/atomics.txt index bba771e..67a27ad 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. -- 2.7.4