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| author | Paolo Bonzini <pbonzini@redhat.com> | 2023-03-03 14:37:51 +0100 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2023-06-06 09:42:14 +0200 |
| commit | 06831001ac8949b0801e0d20c347d97339769a20 (patch) | |
| tree | 01c418078a78ff8049bf5eb7bc36cf3e3bddcb14 /docs | |
| parent | 09a49afeae2542993d4cdc5d7af22abdfce7a3ba (diff) | |
| download | qemu-06831001ac8949b0801e0d20c347d97339769a20.zip qemu-06831001ac8949b0801e0d20c347d97339769a20.tar.gz qemu-06831001ac8949b0801e0d20c347d97339769a20.tar.bz2 | |
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.rst | 27 |
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); | +--------------------------------+ |