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atomics: eliminate mb_read/mb_set

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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>
This commit is contained in:
Paolo Bonzini 2023-03-03 14:37:51 +01:00
parent 09a49afeae
commit 06831001ac
11 changed files with 20 additions and 46 deletions
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27
docs/devel/atomics.rst
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@ -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); |
+--------------------------------+