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| -rw-r--r-- | docs/devel/atomics.rst | 20 |
1 files changed, 13 insertions, 7 deletions
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst index 633df65..81ec26b 100644 --- a/docs/devel/atomics.rst +++ b/docs/devel/atomics.rst @@ -469,13 +469,19 @@ and memory barriers, and the equivalents in QEMU: In QEMU, the second kind is named ``atomic_OP_fetch``. - different atomic read-modify-write operations in Linux imply - a different set of memory barriers; in QEMU, all of them enforce - sequential consistency. - -- in QEMU, ``qatomic_read()`` and ``qatomic_set()`` do not participate in - the ordering enforced by read-modify-write operations. - This is because QEMU uses the C11 memory model. The following example - is correct in Linux but not in QEMU: + a different set of memory barriers. In QEMU, all of them enforce + sequential consistency: there is a single order in which the + program sees them happen. + +- however, according to the C11 memory model that QEMU uses, this order + does not propagate to other memory accesses on either side of the + read-modify-write operation. As far as those are concerned, the + operation consist of just a load-acquire followed by a store-release. + Stores that precede the RMW operation, and loads that follow it, can + still be reordered and will happen *in the middle* of the read-modify-write + operation! + + Therefore, the following example is correct in Linux but not in QEMU: +----------------------------------+--------------------------------+ | Linux (correct) | QEMU (incorrect) | |