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author | Emilio G. Cota <cota@braap.org> | 2016-05-24 16:06:12 -0400 |
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committer | Paolo Bonzini <pbonzini@redhat.com> | 2016-05-29 09:11:11 +0200 |
commit | 56ebe02203f033a8399f7f6ea6972225ed87101c (patch) | |
tree | de17c1e0e38cd4072b4b78be0431d750e201d437 /docs | |
parent | 141af038dd1e73ed32e473046adeb822537c1152 (diff) | |
download | qemu-56ebe02203f033a8399f7f6ea6972225ed87101c.zip qemu-56ebe02203f033a8399f7f6ea6972225ed87101c.tar.gz qemu-56ebe02203f033a8399f7f6ea6972225ed87101c.tar.bz2 |
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 <cota@braap.org>
Message-Id: <1464120374-8950-2-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'docs')
-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 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. |