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authorPaolo Bonzini <pbonzini@redhat.com>2023-03-02 11:10:56 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2023-03-07 12:38:40 +0100
commitff00bed1897c3d27adc5b0cec6f6eeb5a7d13176 (patch)
treec2e747f67b1dc6b45ebdc11be7f7e763ee5ffd71 /docs
parentc61d1a066cb6cf90662c82d0e35660fc0ccacbaf (diff)
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qatomic: add smp_mb__before/after_rmw()
On ARM, seqcst loads and stores (which QEMU does not use) are compiled respectively as LDAR and STLR instructions. Even though LDAR is also used for load-acquire operations, it also waits for all STLRs to leave the store buffer. Thus, LDAR and STLR alone are load-acquire and store-release operations, but LDAR also provides store-against-load ordering as long as the previous store is a STLR. Compare this to ARMv7, where store-release is DMB+STR and load-acquire is LDR+DMB, but an additional DMB is needed between store-seqcst and load-seqcst (e.g. DMB+STR+DMB+LDR+DMB); or with x86, where MOV provides load-acquire and store-release semantics and the two can be reordered. Likewise, on ARM sequentially consistent read-modify-write operations only need to use LDAXR and STLXR respectively for the load and the store, while on x86 they need to use the stronger LOCK prefix. In a strange twist of events, however, the _stronger_ semantics of the ARM instructions can end up causing bugs on ARM, not on x86. The problems occur when seqcst atomics are mixed with relaxed atomics. QEMU's atomics try to bridge the Linux API (that most of the developers are familiar with) and the C11 API, and the two have a substantial difference: - in Linux, strongly-ordered atomics such as atomic_add_return() affect the global ordering of _all_ memory operations, including for example READ_ONCE()/WRITE_ONCE() - in C11, sequentially consistent atomics (except for seq-cst fences) only affect the ordering of sequentially consistent operations. In particular, since relaxed loads are done with LDR on ARM, they are not ordered against seqcst stores (which are done with STLR). QEMU implements high-level synchronization primitives with the idea that the primitives contain the necessary memory barriers, and the callers can use relaxed atomics (qatomic_read/qatomic_set) or even regular accesses. This is very much incompatible with the C11 view that seqcst accesses are only ordered against other seqcst accesses, and requires using seqcst fences as in the following example: qatomic_set(&y, 1); qatomic_set(&x, 1); smp_mb(); smp_mb(); ... qatomic_read(&x) ... ... qatomic_read(&y) ... When a qatomic_*() read-modify write operation is used instead of one or both stores, developers that are more familiar with the Linux API may be tempted to omit the smp_mb(), which will work on x86 but not on ARM. This nasty difference between Linux and C11 read-modify-write operations has already caused issues in util/async.c and more are being found. Provide something similar to Linux smp_mb__before/after_atomic(); this has the double function of documenting clearly why there is a memory barrier, and avoiding a double barrier on x86 and s390x systems. The new macro can already be put to use in qatomic_mb_set(). Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: David Hildenbrand <david@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'docs')
-rw-r--r--docs/devel/atomics.rst26
1 files changed, 21 insertions, 5 deletions
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst
index 7957310..633df65 100644
--- a/docs/devel/atomics.rst
+++ b/docs/devel/atomics.rst
@@ -27,7 +27,8 @@ provides macros that fall in three camps:
- weak atomic access and manual memory barriers: ``qatomic_read()``,
``qatomic_set()``, ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
- ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``;
+ ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``,
+ ``smp_mb__before_rmw()``, ``smp_mb__after_rmw()``;
- sequentially consistent atomic access: everything else.
@@ -472,7 +473,7 @@ and memory barriers, and the equivalents in QEMU:
sequential consistency.
- in QEMU, ``qatomic_read()`` and ``qatomic_set()`` do not participate in
- the total ordering enforced by sequentially-consistent operations.
+ 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:
@@ -488,9 +489,24 @@ and memory barriers, and the equivalents in QEMU:
because the read of ``y`` can be moved (by either the processor or the
compiler) before the write of ``x``.
- Fixing this requires an ``smp_mb()`` memory barrier between the write
- of ``x`` and the read of ``y``. In the common case where only one thread
- writes ``x``, it is also possible to write it like this:
+ Fixing this requires a full memory barrier between the write of ``x`` and
+ the read of ``y``. QEMU provides ``smp_mb__before_rmw()`` and
+ ``smp_mb__after_rmw()``; they act both as an optimization,
+ avoiding the memory barrier on processors where it is unnecessary,
+ and as a clarification of this corner case of the C11 memory model:
+
+ +--------------------------------+
+ | QEMU (correct) |
+ +================================+
+ | :: |
+ | |
+ | a = qatomic_fetch_add(&x, 2);|
+ | smp_mb__after_rmw(); |
+ | b = qatomic_read(&y); |
+ +--------------------------------+
+
+ In the common case where only one thread writes ``x``, it is also possible
+ to write it like this:
+--------------------------------+
| QEMU (correct) |