510 lines
25 KiB
ReStructuredText
510 lines
25 KiB
ReStructuredText
.. _atomics-ref:
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=========================
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Atomic operations in QEMU
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=========================
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CPUs perform independent memory operations effectively in random order.
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but this can be a problem for CPU-CPU interaction (including interactions
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between QEMU and the guest). Multi-threaded programs use various tools
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to instruct the compiler and the CPU to restrict the order to something
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that is consistent with the expectations of the programmer.
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The most basic tool is locking. Mutexes, condition variables and
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semaphores are used in QEMU, and should be the default approach to
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synchronization. Anything else is considerably harder, but it's
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also justified more often than one would like;
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the most performance-critical parts of QEMU in particular require
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a very low level approach to concurrency, involving memory barriers
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and atomic operations. The semantics of concurrent memory accesses are governed
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by the C11 memory model.
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QEMU provides a header, ``qemu/atomic.h``, which wraps C11 atomics to
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provide better portability and a less verbose syntax. ``qemu/atomic.h``
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provides macros that fall in three camps:
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- compiler barriers: ``barrier()``;
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- weak atomic access and manual memory barriers: ``qatomic_read()``,
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``qatomic_set()``, ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
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``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``;
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- sequentially consistent atomic access: everything else.
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In general, use of ``qemu/atomic.h`` should be wrapped with more easily
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used data structures (e.g. the lock-free singly-linked list operations
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``QSLIST_INSERT_HEAD_ATOMIC`` and ``QSLIST_MOVE_ATOMIC``) or synchronization
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primitives (such as RCU, ``QemuEvent`` or ``QemuLockCnt``). Bare use of
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atomic operations and memory barriers should be limited to inter-thread
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checking of flags and documented thoroughly.
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Compiler memory barrier
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=======================
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``barrier()`` prevents the compiler from moving the memory accesses on
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either side of it to the other side. The compiler barrier has no direct
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effect on the CPU, which may then reorder things however it wishes.
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``barrier()`` is mostly used within ``qemu/atomic.h`` itself. On some
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architectures, CPU guarantees are strong enough that blocking compiler
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optimizations already ensures the correct order of execution. In this
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case, ``qemu/atomic.h`` will reduce stronger memory barriers to simple
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compiler barriers.
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Still, ``barrier()`` can be useful when writing code that can be interrupted
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by signal handlers.
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Sequentially consistent atomic access
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=====================================
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Most of the operations in the ``qemu/atomic.h`` header ensure *sequential
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consistency*, where "the result of any execution is the same as if the
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operations of all the processors were executed in some sequential order,
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and the operations of each individual processor appear in this sequence
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in the order specified by its program".
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``qemu/atomic.h`` provides the following set of atomic read-modify-write
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operations::
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void qatomic_inc(ptr)
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void qatomic_dec(ptr)
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void qatomic_add(ptr, val)
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void qatomic_sub(ptr, val)
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void qatomic_and(ptr, val)
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void qatomic_or(ptr, val)
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typeof(*ptr) qatomic_fetch_inc(ptr)
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typeof(*ptr) qatomic_fetch_dec(ptr)
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typeof(*ptr) qatomic_fetch_add(ptr, val)
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typeof(*ptr) qatomic_fetch_sub(ptr, val)
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typeof(*ptr) qatomic_fetch_and(ptr, val)
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typeof(*ptr) qatomic_fetch_or(ptr, val)
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typeof(*ptr) qatomic_fetch_xor(ptr, val)
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typeof(*ptr) qatomic_fetch_inc_nonzero(ptr)
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typeof(*ptr) qatomic_xchg(ptr, val)
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typeof(*ptr) qatomic_cmpxchg(ptr, old, new)
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all of which return the old value of ``*ptr``. These operations are
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polymorphic; they operate on any type that is as wide as a pointer or
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smaller.
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Similar operations return the new value of ``*ptr``::
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typeof(*ptr) qatomic_inc_fetch(ptr)
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typeof(*ptr) qatomic_dec_fetch(ptr)
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typeof(*ptr) qatomic_add_fetch(ptr, val)
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typeof(*ptr) qatomic_sub_fetch(ptr, val)
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typeof(*ptr) qatomic_and_fetch(ptr, val)
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typeof(*ptr) qatomic_or_fetch(ptr, val)
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typeof(*ptr) qatomic_xor_fetch(ptr, val)
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``qemu/atomic.h`` also provides loads and stores that cannot be reordered
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with each other::
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typeof(*ptr) qatomic_mb_read(ptr)
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void qatomic_mb_set(ptr, val)
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However these do not provide sequential consistency and, in particular,
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they do not participate in the total ordering enforced by
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sequentially-consistent operations. For this reason they are deprecated.
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They should instead be replaced with any of the following (ordered from
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easiest to hardest):
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- accesses inside a mutex or spinlock
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- lightweight synchronization primitives such as ``QemuEvent``
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- RCU operations (``qatomic_rcu_read``, ``qatomic_rcu_set``) when publishing
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or accessing a new version of a data structure
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- other atomic accesses: ``qatomic_read`` and ``qatomic_load_acquire`` for
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loads, ``qatomic_set`` and ``qatomic_store_release`` for stores, ``smp_mb``
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to forbid reordering subsequent loads before a store.
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Weak atomic access and manual memory barriers
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=============================================
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Compared to sequentially consistent atomic access, programming with
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weaker consistency models can be considerably more complicated.
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The only guarantees that you can rely upon in this case are:
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- atomic accesses will not cause data races (and hence undefined behavior);
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ordinary accesses instead cause data races if they are concurrent with
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other accesses of which at least one is a write. In order to ensure this,
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the compiler will not optimize accesses out of existence, create unsolicited
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accesses, or perform other similar optimzations.
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- acquire operations will appear to happen, with respect to the other
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components of the system, before all the LOAD or STORE operations
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specified afterwards.
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- release operations will appear to happen, with respect to the other
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components of the system, after all the LOAD or STORE operations
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specified before.
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- release operations will *synchronize with* acquire operations;
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see :ref:`acqrel` for a detailed explanation.
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When using this model, variables are accessed with:
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- ``qatomic_read()`` and ``qatomic_set()``; these prevent the compiler from
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optimizing accesses out of existence and creating unsolicited
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accesses, but do not otherwise impose any ordering on loads and
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stores: both the compiler and the processor are free to reorder
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them.
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- ``qatomic_load_acquire()``, which guarantees the LOAD to appear to
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happen, with respect to the other components of the system,
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before all the LOAD or STORE operations specified afterwards.
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Operations coming before ``qatomic_load_acquire()`` can still be
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reordered after it.
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- ``qatomic_store_release()``, which guarantees the STORE to appear to
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happen, with respect to the other components of the system,
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after all the LOAD or STORE operations specified before.
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Operations coming after ``qatomic_store_release()`` can still be
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reordered before it.
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Restrictions to the ordering of accesses can also be specified
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using the memory barrier macros: ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
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``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``.
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Memory barriers control the order of references to shared memory.
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They come in six kinds:
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- ``smp_rmb()`` guarantees that all the LOAD operations specified before
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the barrier will appear to happen before all the LOAD operations
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specified after the barrier with respect to the other components of
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the system.
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In other words, ``smp_rmb()`` puts a partial ordering on loads, but is not
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required to have any effect on stores.
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- ``smp_wmb()`` guarantees that all the STORE operations specified before
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the barrier will appear to happen before all the STORE operations
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specified after the barrier with respect to the other components of
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the system.
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In other words, ``smp_wmb()`` puts a partial ordering on stores, but is not
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required to have any effect on loads.
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- ``smp_mb_acquire()`` guarantees that all the LOAD operations specified before
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the barrier will appear to happen before all the LOAD or STORE operations
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specified after the barrier with respect to the other components of
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the system.
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- ``smp_mb_release()`` guarantees that all the STORE operations specified *after*
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the barrier will appear to happen after all the LOAD or STORE operations
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specified *before* the barrier with respect to the other components of
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the system.
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- ``smp_mb()`` guarantees that all the LOAD and STORE operations specified
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before the barrier will appear to happen before all the LOAD and
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STORE operations specified after the barrier with respect to the other
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components of the system.
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``smp_mb()`` puts a partial ordering on both loads and stores. It is
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stronger than both a read and a write memory barrier; it implies both
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``smp_mb_acquire()`` and ``smp_mb_release()``, but it also prevents STOREs
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coming before the barrier from overtaking LOADs coming after the
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barrier and vice versa.
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- ``smp_read_barrier_depends()`` is a weaker kind of read barrier. On
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most processors, whenever two loads are performed such that the
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second depends on the result of the first (e.g., the first load
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retrieves the address to which the second load will be directed),
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the processor will guarantee that the first LOAD will appear to happen
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before the second with respect to the other components of the system.
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However, this is not always true---for example, it was not true on
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Alpha processors. Whenever this kind of access happens to shared
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memory (that is not protected by a lock), a read barrier is needed,
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and ``smp_read_barrier_depends()`` can be used instead of ``smp_rmb()``.
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Note that the first load really has to have a _data_ dependency and not
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a control dependency. If the address for the second load is dependent
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on the first load, but the dependency is through a conditional rather
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than actually loading the address itself, then it's a _control_
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dependency and a full read barrier or better is required.
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Memory barriers and ``qatomic_load_acquire``/``qatomic_store_release`` are
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mostly used when a data structure has one thread that is always a writer
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and one thread that is always a reader:
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+----------------------------------+----------------------------------+
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| thread 1 | thread 2 |
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+==================================+==================================+
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| :: | :: |
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| qatomic_store_release(&a, x); | y = qatomic_load_acquire(&b); |
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| qatomic_store_release(&b, y); | x = qatomic_load_acquire(&a); |
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+----------------------------------+----------------------------------+
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In this case, correctness is easy to check for using the "pairing"
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trick that is explained below.
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Sometimes, a thread is accessing many variables that are otherwise
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unrelated to each other (for example because, apart from the current
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thread, exactly one other thread will read or write each of these
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variables). In this case, it is possible to "hoist" the barriers
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outside a loop. For example:
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+------------------------------------------+----------------------------------+
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| before | after |
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+==========================================+==================================+
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| :: | :: |
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| n = 0; | n = 0; |
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| for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
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| n += qatomic_load_acquire(&a[i]); | n += qatomic_read(&a[i]); |
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| | smp_mb_acquire(); |
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+------------------------------------------+----------------------------------+
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| :: | :: |
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| | smp_mb_release(); |
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| for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) |
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| qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); |
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+------------------------------------------+----------------------------------+
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Splitting a loop can also be useful to reduce the number of barriers:
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+------------------------------------------+----------------------------------+
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| before | after |
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+==========================================+==================================+
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| :: | :: |
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| n = 0; | smp_mb_release(); |
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| for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
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| qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); |
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| smp_mb(); | smb_mb(); |
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| n += qatomic_read(&b[i]); | n = 0; |
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| } | for (i = 0; i < 10; i++) |
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| | n += qatomic_read(&b[i]); |
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+------------------------------------------+----------------------------------+
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In this case, a ``smp_mb_release()`` is also replaced with a (possibly cheaper, and clearer
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as well) ``smp_wmb()``:
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+------------------------------------------+----------------------------------+
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| before | after |
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+==========================================+==================================+
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| :: | :: |
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| | smp_mb_release(); |
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| for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) |
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| qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); |
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| qatomic_store_release(&b[i], false); | smb_wmb(); |
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| } | for (i = 0; i < 10; i++) |
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| | qatomic_set(&b[i], false); |
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+------------------------------------------+----------------------------------+
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.. _acqrel:
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Acquire/release pairing and the *synchronizes-with* relation
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------------------------------------------------------------
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Atomic operations other than ``qatomic_set()`` and ``qatomic_read()`` have
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either *acquire* or *release* semantics [#rmw]_. This has two effects:
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.. [#rmw] Read-modify-write operations can have both---acquire applies to the
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read part, and release to the write.
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- within a thread, they are ordered either before subsequent operations
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(for acquire) or after previous operations (for release).
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- if a release operation in one thread *synchronizes with* an acquire operation
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in another thread, the ordering constraints propagates from the first to the
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second thread. That is, everything before the release operation in the
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first thread is guaranteed to *happen before* everything after the
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acquire operation in the second thread.
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The concept of acquire and release semantics is not exclusive to atomic
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operations; almost all higher-level synchronization primitives also have
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acquire or release semantics. For example:
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- ``pthread_mutex_lock`` has acquire semantics, ``pthread_mutex_unlock`` has
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release semantics and synchronizes with a ``pthread_mutex_lock`` for the
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same mutex.
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- ``pthread_cond_signal`` and ``pthread_cond_broadcast`` have release semantics;
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``pthread_cond_wait`` has both release semantics (synchronizing with
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``pthread_mutex_lock``) and acquire semantics (synchronizing with
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``pthread_mutex_unlock`` and signaling of the condition variable).
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- ``pthread_create`` has release semantics and synchronizes with the start
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of the new thread; ``pthread_join`` has acquire semantics and synchronizes
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with the exiting of the thread.
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- ``qemu_event_set`` has release semantics, ``qemu_event_wait`` has
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acquire semantics.
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For example, in the following example there are no atomic accesses, but still
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thread 2 is relying on the *synchronizes-with* relation between ``pthread_exit``
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(release) and ``pthread_join`` (acquire):
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+----------------------+-------------------------------+
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| thread 1 | thread 2 |
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+======================+===============================+
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| :: | :: |
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| *a = 1; | |
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| pthread_exit(a); | pthread_join(thread1, &a); |
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| | x = *a; |
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+----------------------+-------------------------------+
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Synchronization between threads basically descends from this pairing of
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a release operation and an acquire operation. Therefore, atomic operations
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other than ``qatomic_set()`` and ``qatomic_read()`` will almost always be
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paired with another operation of the opposite kind: an acquire operation
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will pair with a release operation and vice versa. This rule of thumb is
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extremely useful; in the case of QEMU, however, note that the other
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operation may actually be in a driver that runs in the guest!
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``smp_read_barrier_depends()``, ``smp_rmb()``, ``smp_mb_acquire()``,
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``qatomic_load_acquire()`` and ``qatomic_rcu_read()`` all count
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as acquire operations. ``smp_wmb()``, ``smp_mb_release()``,
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``qatomic_store_release()`` and ``qatomic_rcu_set()`` all count as release
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operations. ``smp_mb()`` counts as both acquire and release, therefore
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it can pair with any other atomic operation. Here is an example:
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+----------------------+------------------------------+
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| thread 1 | thread 2 |
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+======================+==============================+
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| :: | :: |
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| qatomic_set(&a, 1);| |
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| smp_wmb(); | |
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| qatomic_set(&b, 2);| x = qatomic_read(&b); |
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| | smp_rmb(); |
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| | y = qatomic_read(&a); |
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+----------------------+------------------------------+
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Note that a load-store pair only counts if the two operations access the
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same variable: that is, a store-release on a variable ``x`` *synchronizes
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with* a load-acquire on a variable ``x``, while a release barrier
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synchronizes with any acquire operation. The following example shows
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correct synchronization:
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+--------------------------------+--------------------------------+
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| thread 1 | thread 2 |
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+================================+================================+
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| :: | :: |
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| qatomic_set(&a, 1); | |
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| qatomic_store_release(&b, 2);| x = qatomic_load_acquire(&b);|
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| | y = qatomic_read(&a); |
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+--------------------------------+--------------------------------+
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Acquire and release semantics of higher-level primitives can also be
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relied upon for the purpose of establishing the *synchronizes with*
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relation.
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Note that the "writing" thread is accessing the variables in the
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opposite order as the "reading" thread. This is expected: stores
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before a release operation will normally match the loads after
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the acquire operation, and vice versa. In fact, this happened already
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in the ``pthread_exit``/``pthread_join`` example above.
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Finally, this more complex example has more than two accesses and data
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dependency barriers. It also does not use atomic accesses whenever there
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cannot be a data race:
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+----------------------+------------------------------+
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| thread 1 | thread 2 |
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+======================+==============================+
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| :: | :: |
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| b[2] = 1; | |
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| smp_wmb(); | |
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| x->i = 2; | |
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| smp_wmb(); | |
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| qatomic_set(&a, x);| x = qatomic_read(&a); |
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| | smp_read_barrier_depends(); |
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| | y = x->i; |
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| | smp_read_barrier_depends(); |
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| | z = b[y]; |
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+----------------------+------------------------------+
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Comparison with Linux kernel primitives
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=======================================
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Here is a list of differences between Linux kernel atomic operations
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and memory barriers, and the equivalents in QEMU:
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- atomic operations in Linux are always on a 32-bit int type and
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use a boxed ``atomic_t`` type; atomic operations in QEMU are polymorphic
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and use normal C types.
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- Originally, ``atomic_read`` and ``atomic_set`` in Linux gave no guarantee
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at all. Linux 4.1 updated them to implement volatile
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semantics via ``ACCESS_ONCE`` (or the more recent ``READ``/``WRITE_ONCE``).
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QEMU's ``qatomic_read`` and ``qatomic_set`` implement C11 atomic relaxed
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semantics if the compiler supports it, and volatile semantics otherwise.
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Both semantics prevent the compiler from doing certain transformations;
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the difference is that atomic accesses are guaranteed to be atomic,
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while volatile accesses aren't. Thus, in the volatile case we just cross
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our fingers hoping that the compiler will generate atomic accesses,
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since we assume the variables passed are machine-word sized and
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properly aligned.
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No barriers are implied by ``qatomic_read`` and ``qatomic_set`` in either
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Linux or QEMU.
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- atomic read-modify-write operations in Linux are of three kinds:
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===================== =========================================
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``atomic_OP`` returns void
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``atomic_OP_return`` returns new value of the variable
|
|
``atomic_fetch_OP`` returns the old value of the variable
|
|
``atomic_cmpxchg`` returns the old value of the variable
|
|
===================== =========================================
|
|
|
|
In QEMU, the second kind is named ``atomic_OP_fetch``.
|
|
|
|
- different atomic read-modify-write operations in Linux imply
|
|
a different set of memory barriers; in QEMU, all of them enforce
|
|
sequential consistency.
|
|
|
|
- in QEMU, ``qatomic_read()`` and ``qatomic_set()`` do not participate in
|
|
the total ordering enforced by sequentially-consistent operations.
|
|
This is because QEMU uses the C11 memory model. The following example
|
|
is correct in Linux but not in QEMU:
|
|
|
|
+----------------------------------+--------------------------------+
|
|
| Linux (correct) | QEMU (incorrect) |
|
|
+==================================+================================+
|
|
| :: | :: |
|
|
| | |
|
|
| a = atomic_fetch_add(&x, 2); | a = qatomic_fetch_add(&x, 2);|
|
|
| b = READ_ONCE(&y); | b = qatomic_read(&y); |
|
|
+----------------------------------+--------------------------------+
|
|
|
|
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:
|
|
|
|
+--------------------------------+
|
|
| QEMU (correct) |
|
|
+================================+
|
|
| :: |
|
|
| |
|
|
| a = qatomic_read(&x); |
|
|
| qatomic_set(&x, a + 2); |
|
|
| smp_mb(); |
|
|
| b = qatomic_read(&y); |
|
|
+--------------------------------+
|
|
|
|
Sources
|
|
=======
|
|
|
|
- ``Documentation/memory-barriers.txt`` from the Linux kernel
|