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authorPaolo Bonzini <pbonzini@redhat.com>2023-03-03 14:37:51 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2023-06-06 09:42:14 +0200
commit06831001ac8949b0801e0d20c347d97339769a20 (patch)
tree01c418078a78ff8049bf5eb7bc36cf3e3bddcb14 /docs
parent09a49afeae2542993d4cdc5d7af22abdfce7a3ba (diff)
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atomics: eliminate mb_read/mb_set
qatomic_mb_read and qatomic_mb_set were the very first atomic primitives introduced for QEMU; their semantics are unclear and they provide a false sense of safety. The last use of qatomic_mb_read() has been removed, so delete it. qatomic_mb_set() instead can survive as an optimized qatomic_set()+smp_mb(), similar to Linux's smp_store_mb(), but rename it to qatomic_set_mb() to match the order of the two operations. Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'docs')
-rw-r--r--docs/devel/atomics.rst27
1 files changed, 4 insertions, 23 deletions
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst
index 2480763..ff9b5ee 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); |
+--------------------------------+