| .. _atomics-ref: |
| |
| ========================= |
| Atomic operations in QEMU |
| ========================= |
| |
| CPUs perform independent memory operations effectively in random order. |
| but this can be a problem for CPU-CPU interaction (including interactions |
| between QEMU and the guest). Multi-threaded programs use various tools |
| to instruct the compiler and the CPU to restrict the order to something |
| that is consistent with the expectations of the programmer. |
| |
| The most basic tool is locking. Mutexes, condition variables and |
| semaphores are used in QEMU, and should be the default approach to |
| synchronization. Anything else is considerably harder, but it's |
| also justified more often than one would like; |
| the most performance-critical parts of QEMU in particular require |
| a very low level approach to concurrency, involving memory barriers |
| and atomic operations. The semantics of concurrent memory accesses are governed |
| by the C11 memory model. |
| |
| QEMU provides a header, ``qemu/atomic.h``, which wraps C11 atomics to |
| provide better portability and a less verbose syntax. ``qemu/atomic.h`` |
| provides macros that fall in three camps: |
| |
| - compiler barriers: ``barrier()``; |
| |
| - 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__before_rmw()``, ``smp_mb__after_rmw()``; |
| |
| - sequentially consistent atomic access: everything else. |
| |
| In general, use of ``qemu/atomic.h`` should be wrapped with more easily |
| used data structures (e.g. the lock-free singly-linked list operations |
| ``QSLIST_INSERT_HEAD_ATOMIC`` and ``QSLIST_MOVE_ATOMIC``) or synchronization |
| primitives (such as RCU, ``QemuEvent`` or ``QemuLockCnt``). Bare use of |
| atomic operations and memory barriers should be limited to inter-thread |
| checking of flags and documented thoroughly. |
| |
| |
| |
| Compiler memory barrier |
| ======================= |
| |
| ``barrier()`` prevents the compiler from moving the memory accesses on |
| either side of it to the other side. The compiler barrier has no direct |
| effect on the CPU, which may then reorder things however it wishes. |
| |
| ``barrier()`` is mostly used within ``qemu/atomic.h`` itself. On some |
| architectures, CPU guarantees are strong enough that blocking compiler |
| optimizations already ensures the correct order of execution. In this |
| case, ``qemu/atomic.h`` will reduce stronger memory barriers to simple |
| compiler barriers. |
| |
| Still, ``barrier()`` can be useful when writing code that can be interrupted |
| by signal handlers. |
| |
| |
| Sequentially consistent atomic access |
| ===================================== |
| |
| Most of the operations in the ``qemu/atomic.h`` header ensure *sequential |
| consistency*, where "the result of any execution is the same as if the |
| operations of all the processors were executed in some sequential order, |
| and the operations of each individual processor appear in this sequence |
| in the order specified by its program". |
| |
| ``qemu/atomic.h`` provides the following set of atomic read-modify-write |
| operations:: |
| |
| void qatomic_inc(ptr) |
| void qatomic_dec(ptr) |
| void qatomic_add(ptr, val) |
| void qatomic_sub(ptr, val) |
| void qatomic_and(ptr, val) |
| void qatomic_or(ptr, val) |
| |
| typeof(*ptr) qatomic_fetch_inc(ptr) |
| typeof(*ptr) qatomic_fetch_dec(ptr) |
| typeof(*ptr) qatomic_fetch_add(ptr, val) |
| typeof(*ptr) qatomic_fetch_sub(ptr, val) |
| typeof(*ptr) qatomic_fetch_and(ptr, val) |
| typeof(*ptr) qatomic_fetch_or(ptr, val) |
| typeof(*ptr) qatomic_fetch_xor(ptr, val) |
| typeof(*ptr) qatomic_fetch_inc_nonzero(ptr) |
| typeof(*ptr) qatomic_xchg(ptr, val) |
| typeof(*ptr) qatomic_cmpxchg(ptr, old, new) |
| |
| all of which return the old value of ``*ptr``. These operations are |
| polymorphic; they operate on any type that is as wide as a pointer or |
| smaller. |
| |
| Similar operations return the new value of ``*ptr``:: |
| |
| typeof(*ptr) qatomic_inc_fetch(ptr) |
| typeof(*ptr) qatomic_dec_fetch(ptr) |
| typeof(*ptr) qatomic_add_fetch(ptr, val) |
| typeof(*ptr) qatomic_sub_fetch(ptr, val) |
| typeof(*ptr) qatomic_and_fetch(ptr, val) |
| typeof(*ptr) qatomic_or_fetch(ptr, val) |
| typeof(*ptr) qatomic_xor_fetch(ptr, val) |
| |
| ``qemu/atomic.h`` also provides an optimized shortcut for |
| ``qatomic_set`` followed by ``smp_mb``:: |
| |
| void qatomic_set_mb(ptr, val) |
| |
| |
| Weak atomic access and manual memory barriers |
| ============================================= |
| |
| Compared to sequentially consistent atomic access, programming with |
| weaker consistency models can be considerably more complicated. |
| The only guarantees that you can rely upon in this case are: |
| |
| - atomic accesses will not cause data races (and hence undefined behavior); |
| ordinary accesses instead cause data races if they are concurrent with |
| other accesses of which at least one is a write. In order to ensure this, |
| the compiler will not optimize accesses out of existence, create unsolicited |
| accesses, or perform other similar optimzations. |
| |
| - acquire operations will appear to happen, with respect to the other |
| components of the system, before all the LOAD or STORE operations |
| specified afterwards. |
| |
| - release operations will appear to happen, with respect to the other |
| components of the system, after all the LOAD or STORE operations |
| specified before. |
| |
| - release operations will *synchronize with* acquire operations; |
| see :ref:`acqrel` for a detailed explanation. |
| |
| When using this model, variables are accessed with: |
| |
| - ``qatomic_read()`` and ``qatomic_set()``; these prevent the compiler from |
| optimizing accesses out of existence and creating unsolicited |
| accesses, but do not otherwise impose any ordering on loads and |
| stores: both the compiler and the processor are free to reorder |
| them. |
| |
| - ``qatomic_load_acquire()``, which guarantees the LOAD to appear to |
| happen, with respect to the other components of the system, |
| before all the LOAD or STORE operations specified afterwards. |
| Operations coming before ``qatomic_load_acquire()`` can still be |
| reordered after it. |
| |
| - ``qatomic_store_release()``, which guarantees the STORE to appear to |
| happen, with respect to the other components of the system, |
| after all the LOAD or STORE operations specified before. |
| Operations coming after ``qatomic_store_release()`` can still be |
| reordered before it. |
| |
| Restrictions to the ordering of accesses can also be specified |
| using the memory barrier macros: ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``, |
| ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``. |
| |
| Memory barriers control the order of references to shared memory. |
| They come in six kinds: |
| |
| - ``smp_rmb()`` guarantees that all the LOAD operations specified before |
| the barrier will appear to happen before all the LOAD operations |
| specified after the barrier with respect to the other components of |
| the system. |
| |
| In other words, ``smp_rmb()`` puts a partial ordering on loads, but is not |
| required to have any effect on stores. |
| |
| - ``smp_wmb()`` guarantees that all the STORE operations specified before |
| the barrier will appear to happen before all the STORE operations |
| specified after the barrier with respect to the other components of |
| the system. |
| |
| In other words, ``smp_wmb()`` puts a partial ordering on stores, but is not |
| required to have any effect on loads. |
| |
| - ``smp_mb_acquire()`` guarantees that all the LOAD operations specified before |
| the barrier will appear to happen before all the LOAD or STORE operations |
| specified after the barrier with respect to the other components of |
| the system. |
| |
| - ``smp_mb_release()`` guarantees that all the STORE operations specified *after* |
| the barrier will appear to happen after all the LOAD or STORE operations |
| specified *before* the barrier with respect to the other components of |
| the system. |
| |
| - ``smp_mb()`` guarantees that all the LOAD and STORE operations specified |
| before the barrier will appear to happen before all the LOAD and |
| STORE operations specified after the barrier with respect to the other |
| components of the system. |
| |
| ``smp_mb()`` puts a partial ordering on both loads and stores. It is |
| stronger than both a read and a write memory barrier; it implies both |
| ``smp_mb_acquire()`` and ``smp_mb_release()``, but it also prevents STOREs |
| coming before the barrier from overtaking LOADs coming after the |
| barrier and vice versa. |
| |
| - ``smp_read_barrier_depends()`` is a weaker kind of read barrier. On |
| most processors, whenever two loads are performed such that the |
| second depends on the result of the first (e.g., the first load |
| retrieves the address to which the second load will be directed), |
| the processor will guarantee that the first LOAD will appear to happen |
| before the second with respect to the other components of the system. |
| Therefore, unlike ``smp_rmb()`` or ``qatomic_load_acquire()``, |
| ``smp_read_barrier_depends()`` can be just a compiler barrier on |
| weakly-ordered architectures such as Arm or PPC[#]_. |
| |
| Note that the first load really has to have a _data_ dependency and not |
| a control dependency. If the address for the second load is dependent |
| on the first load, but the dependency is through a conditional rather |
| than actually loading the address itself, then it's a _control_ |
| dependency and a full read barrier or better is required. |
| |
| .. [#] The DEC Alpha is an exception, because ``smp_read_barrier_depends()`` |
| needs a processor barrier. On strongly-ordered architectures such |
| as x86 or s390, ``smp_rmb()`` and ``qatomic_load_acquire()`` can |
| also be compiler barriers only. |
| |
| Memory barriers and ``qatomic_load_acquire``/``qatomic_store_release`` are |
| mostly used when a data structure has one thread that is always a writer |
| and one thread that is always a reader: |
| |
| +----------------------------------+----------------------------------+ |
| | thread 1 | thread 2 | |
| +==================================+==================================+ |
| | :: | :: | |
| | | | |
| | qatomic_store_release(&a, x); | y = qatomic_load_acquire(&b); | |
| | qatomic_store_release(&b, y); | x = qatomic_load_acquire(&a); | |
| +----------------------------------+----------------------------------+ |
| |
| In this case, correctness is easy to check for using the "pairing" |
| trick that is explained below. |
| |
| Sometimes, a thread is accessing many variables that are otherwise |
| unrelated to each other (for example because, apart from the current |
| thread, exactly one other thread will read or write each of these |
| variables). In this case, it is possible to "hoist" the barriers |
| outside a loop. For example: |
| |
| +------------------------------------------+----------------------------------+ |
| | before | after | |
| +==========================================+==================================+ |
| | :: | :: | |
| | | | |
| | n = 0; | n = 0; | |
| | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) | |
| | n += qatomic_load_acquire(&a[i]); | n += qatomic_read(&a[i]); | |
| | | smp_mb_acquire(); | |
| +------------------------------------------+----------------------------------+ |
| | :: | :: | |
| | | | |
| | | smp_mb_release(); | |
| | for (i = 0; i < 10; i++) | for (i = 0; i < 10; i++) | |
| | qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); | |
| +------------------------------------------+----------------------------------+ |
| |
| Splitting a loop can also be useful to reduce the number of barriers: |
| |
| +------------------------------------------+----------------------------------+ |
| | before | after | |
| +==========================================+==================================+ |
| | :: | :: | |
| | | | |
| | n = 0; | smp_mb_release(); | |
| | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) | |
| | qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); | |
| | smp_mb(); | smb_mb(); | |
| | n += qatomic_read(&b[i]); | n = 0; | |
| | } | for (i = 0; i < 10; i++) | |
| | | n += qatomic_read(&b[i]); | |
| +------------------------------------------+----------------------------------+ |
| |
| In this case, a ``smp_mb_release()`` is also replaced with a (possibly cheaper, and clearer |
| as well) ``smp_wmb()``: |
| |
| +------------------------------------------+----------------------------------+ |
| | before | after | |
| +==========================================+==================================+ |
| | :: | :: | |
| | | | |
| | | smp_mb_release(); | |
| | for (i = 0; i < 10; i++) { | for (i = 0; i < 10; i++) | |
| | qatomic_store_release(&a[i], false); | qatomic_set(&a[i], false); | |
| | qatomic_store_release(&b[i], false); | smb_wmb(); | |
| | } | for (i = 0; i < 10; i++) | |
| | | qatomic_set(&b[i], false); | |
| +------------------------------------------+----------------------------------+ |
| |
| |
| .. _acqrel: |
| |
| Acquire/release pairing and the *synchronizes-with* relation |
| ------------------------------------------------------------ |
| |
| Atomic operations other than ``qatomic_set()`` and ``qatomic_read()`` have |
| either *acquire* or *release* semantics [#rmw]_. This has two effects: |
| |
| .. [#rmw] Read-modify-write operations can have both---acquire applies to the |
| read part, and release to the write. |
| |
| - within a thread, they are ordered either before subsequent operations |
| (for acquire) or after previous operations (for release). |
| |
| - if a release operation in one thread *synchronizes with* an acquire operation |
| in another thread, the ordering constraints propagates from the first to the |
| second thread. That is, everything before the release operation in the |
| first thread is guaranteed to *happen before* everything after the |
| acquire operation in the second thread. |
| |
| The concept of acquire and release semantics is not exclusive to atomic |
| operations; almost all higher-level synchronization primitives also have |
| acquire or release semantics. For example: |
| |
| - ``pthread_mutex_lock`` has acquire semantics, ``pthread_mutex_unlock`` has |
| release semantics and synchronizes with a ``pthread_mutex_lock`` for the |
| same mutex. |
| |
| - ``pthread_cond_signal`` and ``pthread_cond_broadcast`` have release semantics; |
| ``pthread_cond_wait`` has both release semantics (synchronizing with |
| ``pthread_mutex_lock``) and acquire semantics (synchronizing with |
| ``pthread_mutex_unlock`` and signaling of the condition variable). |
| |
| - ``pthread_create`` has release semantics and synchronizes with the start |
| of the new thread; ``pthread_join`` has acquire semantics and synchronizes |
| with the exiting of the thread. |
| |
| - ``qemu_event_set`` has release semantics, ``qemu_event_wait`` has |
| acquire semantics. |
| |
| For example, in the following example there are no atomic accesses, but still |
| thread 2 is relying on the *synchronizes-with* relation between ``pthread_exit`` |
| (release) and ``pthread_join`` (acquire): |
| |
| +----------------------+-------------------------------+ |
| | thread 1 | thread 2 | |
| +======================+===============================+ |
| | :: | :: | |
| | | | |
| | *a = 1; | | |
| | pthread_exit(a); | pthread_join(thread1, &a); | |
| | | x = *a; | |
| +----------------------+-------------------------------+ |
| |
| Synchronization between threads basically descends from this pairing of |
| a release operation and an acquire operation. Therefore, atomic operations |
| other than ``qatomic_set()`` and ``qatomic_read()`` will almost always be |
| paired with another operation of the opposite kind: an acquire operation |
| will pair with a release operation and vice versa. This rule of thumb is |
| extremely useful; in the case of QEMU, however, note that the other |
| operation may actually be in a driver that runs in the guest! |
| |
| ``smp_read_barrier_depends()``, ``smp_rmb()``, ``smp_mb_acquire()``, |
| ``qatomic_load_acquire()`` and ``qatomic_rcu_read()`` all count |
| as acquire operations. ``smp_wmb()``, ``smp_mb_release()``, |
| ``qatomic_store_release()`` and ``qatomic_rcu_set()`` all count as release |
| operations. ``smp_mb()`` counts as both acquire and release, therefore |
| it can pair with any other atomic operation. Here is an example: |
| |
| +----------------------+------------------------------+ |
| | thread 1 | thread 2 | |
| +======================+==============================+ |
| | :: | :: | |
| | | | |
| | qatomic_set(&a, 1);| | |
| | smp_wmb(); | | |
| | qatomic_set(&b, 2);| x = qatomic_read(&b); | |
| | | smp_rmb(); | |
| | | y = qatomic_read(&a); | |
| +----------------------+------------------------------+ |
| |
| Note that a load-store pair only counts if the two operations access the |
| same variable: that is, a store-release on a variable ``x`` *synchronizes |
| with* a load-acquire on a variable ``x``, while a release barrier |
| synchronizes with any acquire operation. The following example shows |
| correct synchronization: |
| |
| +--------------------------------+--------------------------------+ |
| | thread 1 | thread 2 | |
| +================================+================================+ |
| | :: | :: | |
| | | | |
| | qatomic_set(&a, 1); | | |
| | qatomic_store_release(&b, 2);| x = qatomic_load_acquire(&b);| |
| | | y = qatomic_read(&a); | |
| +--------------------------------+--------------------------------+ |
| |
| Acquire and release semantics of higher-level primitives can also be |
| relied upon for the purpose of establishing the *synchronizes with* |
| relation. |
| |
| Note that the "writing" thread is accessing the variables in the |
| opposite order as the "reading" thread. This is expected: stores |
| before a release operation will normally match the loads after |
| the acquire operation, and vice versa. In fact, this happened already |
| in the ``pthread_exit``/``pthread_join`` example above. |
| |
| Finally, this more complex example has more than two accesses and data |
| dependency barriers. It also does not use atomic accesses whenever there |
| cannot be a data race: |
| |
| +----------------------+------------------------------+ |
| | thread 1 | thread 2 | |
| +======================+==============================+ |
| | :: | :: | |
| | | | |
| | b[2] = 1; | | |
| | smp_wmb(); | | |
| | x->i = 2; | | |
| | smp_wmb(); | | |
| | qatomic_set(&a, x);| x = qatomic_read(&a); | |
| | | smp_read_barrier_depends(); | |
| | | y = x->i; | |
| | | smp_read_barrier_depends(); | |
| | | z = b[y]; | |
| +----------------------+------------------------------+ |
| |
| Comparison with Linux kernel primitives |
| ======================================= |
| |
| Here is a list of differences between Linux kernel atomic operations |
| and memory barriers, and the equivalents in QEMU: |
| |
| - atomic operations in Linux are always on a 32-bit int type and |
| use a boxed ``atomic_t`` type; atomic operations in QEMU are polymorphic |
| and use normal C types. |
| |
| - Originally, ``atomic_read`` and ``atomic_set`` in Linux gave no guarantee |
| at all. Linux 4.1 updated them to implement volatile |
| semantics via ``ACCESS_ONCE`` (or the more recent ``READ``/``WRITE_ONCE``). |
| |
| QEMU's ``qatomic_read`` and ``qatomic_set`` implement C11 atomic relaxed |
| semantics if the compiler supports it, and volatile semantics otherwise. |
| Both semantics prevent the compiler from doing certain transformations; |
| the difference is that atomic accesses are guaranteed to be atomic, |
| while volatile accesses aren't. Thus, in the volatile case we just cross |
| our fingers hoping that the compiler will generate atomic accesses, |
| since we assume the variables passed are machine-word sized and |
| properly aligned. |
| |
| No barriers are implied by ``qatomic_read`` and ``qatomic_set`` in either |
| Linux or QEMU. |
| |
| - atomic read-modify-write operations in Linux are of three kinds: |
| |
| ===================== ========================================= |
| ``atomic_OP`` returns void |
| ``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: there is a single order in which the |
| program sees them happen. |
| |
| - however, according to the C11 memory model that QEMU uses, this order |
| does not propagate to other memory accesses on either side of the |
| read-modify-write operation. As far as those are concerned, the |
| operation consist of just a load-acquire followed by a store-release. |
| Stores that precede the RMW operation, and loads that follow it, can |
| still be reordered and will happen *in the middle* of the read-modify-write |
| operation! |
| |
| Therefore, 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 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) | |
| +================================+ |
| | :: | |
| | | |
| | a = qatomic_read(&x); | |
| | qatomic_set_mb(&x, a + 2); | |
| | b = qatomic_read(&y); | |
| +--------------------------------+ |
| |
| Sources |
| ======= |
| |
| - ``Documentation/memory-barriers.txt`` from the Linux kernel |
