diff options
Diffstat (limited to 'docs/devel/atomics.rst')
| -rw-r--r-- | docs/devel/atomics.rst | 27 |
1 files changed, 4 insertions, 23 deletions
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst index 248076375b..ff9b5ee30c 100644 --- a/docs/devel/atomics.rst +++ b/docs/devel/atomics.rst @@ -102,28 +102,10 @@ Similar operations return the new value of ``*ptr``:: typeof(*ptr) qatomic_or_fetch(ptr, val) typeof(*ptr) qatomic_xor_fetch(ptr, val) -``qemu/atomic.h`` also provides loads and stores that cannot be reordered -with each other:: +``qemu/atomic.h`` also provides an optimized shortcut for +``qatomic_set`` followed by ``smp_mb``:: - typeof(*ptr) qatomic_mb_read(ptr) - void qatomic_mb_set(ptr, val) - -However these do not provide sequential consistency and, in particular, -they do not participate in the total ordering enforced by -sequentially-consistent operations. For this reason they are deprecated. -They should instead be replaced with any of the following (ordered from -easiest to hardest): - -- accesses inside a mutex or spinlock - -- lightweight synchronization primitives such as ``QemuEvent`` - -- RCU operations (``qatomic_rcu_read``, ``qatomic_rcu_set``) when publishing - or accessing a new version of a data structure - -- other atomic accesses: ``qatomic_read`` and ``qatomic_load_acquire`` for - loads, ``qatomic_set`` and ``qatomic_store_release`` for stores, ``smp_mb`` - to forbid reordering subsequent loads before a store. + void qatomic_set_mb(ptr, val) Weak atomic access and manual memory barriers @@ -523,8 +505,7 @@ and memory barriers, and the equivalents in QEMU: | :: | | | | a = qatomic_read(&x); | - | qatomic_set(&x, a + 2); | - | smp_mb(); | + | qatomic_set_mb(&x, a + 2); | | b = qatomic_read(&y); | +--------------------------------+ |