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[Qemu-devel] [PATCH v2 1/3] docs/atomics: update atomic_read/set compari
From: |
Emilio G. Cota |
Subject: |
[Qemu-devel] [PATCH v2 1/3] docs/atomics: update atomic_read/set comparison with Linux |
Date: |
Tue, 24 May 2016 16:06:12 -0400 |
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 <address@hidden>
---
docs/atomics.txt | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/docs/atomics.txt b/docs/atomics.txt
index ef285e3..7540990 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 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.
--
2.5.0