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* Fix missing memory barriers

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* Fix comments about memory ordering
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Merge tag 'for-upstream-mb' of https://gitlab.com/bonzini/qemu into staging

* Fix missing memory barriers
* Fix comments about memory ordering

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* tag 'for-upstream-mb' of https://gitlab.com/bonzini/qemu:
  async: clarify usage of barriers in the polling case
  async: update documentation of the memory barriers
  physmem: add missing memory barrier
  qemu-coroutine-lock: add smp_mb__after_rmw()
  aio-wait: switch to smp_mb__after_rmw()
  edu: add smp_mb__after_rmw()
  qemu-thread-win32: cleanup, fix, document QemuEvent
  qemu-thread-posix: cleanup, fix, document QemuEvent
  qatomic: add smp_mb__before/after_rmw()

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Peter Maydell 2023-03-07 17:02:06 +00:00
commit 7b0f0aa55f
9 changed files with 186 additions and 70 deletions
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26
docs/devel/atomics.rst
View file

@ -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) |