* int8_t atomic_int8_t (*)
* int16_t atomic_int16_t (*)
* int32_t atomic_int32_t (*)
- * uint64_t atomic_uint64_t (*)
- * int64_t atomic_int64_t (*)
*
* (*) Not specified by C11.
*
* ...
* atomic_init(&ai, 123);
*
- * C11 does not hav an destruction function for atomic types, but some
- * implementations of the OVS atomics do need them. Thus, the following
- * function is provided for destroying non-static atomic objects (A is any
- * atomic type):
- *
- * void atomic_destroy(A *object);
- *
- * Destroys 'object'.
- *
*
* Barriers
* ========
*
* memory_order_relaxed:
*
- * Compiler barrier only. Does not imply any CPU memory ordering.
+ * Only atomicity is provided, does not imply any memory ordering with
+ * respect to any other variable (atomic or not). Relaxed accesses to
+ * the same atomic variable will be performed in the program order.
+ * The compiler and CPU are free to move memory accesses to other
+ * variables past the atomic operation.
+ *
+ * memory_order_consume:
+ *
+ * Memory accesses with data dependency on the result of the consume
+ * operation (atomic_read_explicit, or a load operation preceding a
+ * atomic_thread_fence) will not be moved prior to the consume
+ * barrier. Non-data-dependent loads and stores can be reordered to
+ * happen before the consume barrier.
+ *
+ * RCU is the prime example of the use of the consume barrier: The
+ * consume barrier guarantees that reads from a RCU protected object
+ * are performed after the RCU protected pointer is read. A
+ * corresponding release barrier is used to store the modified RCU
+ * protected pointer after the RCU protected object has been fully
+ * constructed. The synchronization between these barriers prevents
+ * the RCU "consumer" from seeing uninitialized data.
+ *
+ * May not be used with atomic_store_explicit(), as consume semantics
+ * applies only to atomic loads.
*
* memory_order_acquire:
*
* barrier. Memory accesses before an acquire barrier *can* be moved
* after it.
*
+ * An atomic_thread_fence with memory_order_acquire does not have a
+ * load operation by itself; it prevents all following memory accesses
+ * from moving prior to preceding loads.
+ *
+ * May not be used with atomic_store_explicit(), as acquire semantics
+ * applies only to atomic loads.
+ *
* memory_order_release:
*
* Memory accesses before a release barrier cannot be moved after the
* barrier. Memory accesses after a release barrier *can* be moved
* before it.
*
+ * An atomic_thread_fence with memory_order_release does not have a
+ * store operation by itself; it prevents all preceding memory accesses
+ * from moving past subsequent stores.
+ *
+ * May not be used with atomic_read_explicit(), as release semantics
+ * applies only to atomic stores.
+ *
* memory_order_acq_rel:
*
* Memory accesses cannot be moved across an acquire-release barrier in
* either direction.
*
+ * May only be used with atomic read-modify-write operations, as both
+ * load and store operation is required for acquire-release semantics.
+ *
+ * An atomic_thread_fence with memory_order_acq_rel does not have
+ * either load or store operation by itself; it prevents all following
+ * memory accesses from moving prior to preceding loads and all
+ * preceding memory accesses from moving past subsequent stores.
+ *
* memory_order_seq_cst:
*
* Prevents movement of memory accesses like an acquire-release barrier,
- * but whereas acquire-release synchronizes cooperating threads,
- * sequential-consistency synchronizes the whole system.
- *
- * memory_order_consume:
+ * but whereas acquire-release synchronizes cooperating threads (using
+ * the same atomic variable), sequential-consistency synchronizes the
+ * whole system, providing a total order for stores on all atomic
+ * variables.
*
- * A slight relaxation of memory_order_acquire.
+ * OVS atomics require the memory_order to be passed as a compile-time constant
+ * value, as some compiler implementations may perform poorly if the memory
+ * order parameter is passed in as a run-time value.
*
* The following functions insert explicit barriers. Most of the other atomic
* functions also include barriers.
* In this section, A is an atomic type and C is the corresponding non-atomic
* type.
*
- * The "store" primitives match C11:
+ * The "store" and "compare_exchange" primitives match C11:
*
* void atomic_store(A *object, C value);
* void atomic_store_explicit(A *object, C value, memory_order);
* Atomically stores 'value' into '*object', respecting the given
* memory order (or memory_order_seq_cst for atomic_store()).
*
+ * bool atomic_compare_exchange_strong(A *object, C *expected, C desired);
+ * bool atomic_compare_exchange_weak(A *object, C *expected, C desired);
+ * bool atomic_compare_exchange_strong_explicit(A *object, C *expected,
+ * C desired,
+ * memory_order success,
+ * memory_order failure);
+ * bool atomic_compare_exchange_weak_explicit(A *object, C *expected,
+ * C desired,
+ * memory_order success,
+ * memory_order failure);
+ *
+ * Atomically loads '*object' and compares it with '*expected' and if
+ * equal, stores 'desired' into '*object' (an atomic read-modify-write
+ * operation) and returns true, and if non-equal, stores the actual
+ * value of '*object' into '*expected' (an atomic load operation) and
+ * returns false. The memory order for the successful case (atomic
+ * read-modify-write operation) is 'success', and for the unsuccessful
+ * case (atomic load operation) 'failure'. 'failure' may not be
+ * stronger than 'success'.
+ *
+ * The weak forms may fail (returning false) also when '*object' equals
+ * '*expected'. The strong form can be implemented by the weak form in
+ * a loop. Some platforms can implement the weak form more
+ * efficiently, so it should be used if the application will need to
+ * loop anyway.
+ *
* The following primitives differ from the C11 ones (and have different names)
* because there does not appear to be a way to implement the standard
* primitives in standard C:
* ATOMIC_FLAG_INIT is an initializer for atomic_flag. The initial state is
* "clear".
*
- * C11 does not have an initialization or destruction function for atomic_flag,
- * because implementations should not need one (one may simply
- * atomic_flag_clear() an uninitialized atomic_flag), but some implementations
- * of the OVS atomics do need them. Thus, the following two functions are
- * provided for initializing and destroying non-static atomic_flags:
- *
- * void atomic_flag_init(volatile atomic_flag *object);
- *
- * Initializes 'object'. The initial state is "clear".
- *
- * void atomic_flag_destroy(volatile atomic_flag *object);
- *
- * Destroys 'object'.
+ * An atomic_flag may also be initialized at runtime with atomic_flag_clear().
*
*
* Operations
#if __CHECKER__
/* sparse doesn't understand some GCC extensions we use. */
#include "ovs-atomic-pthreads.h"
- #elif HAVE_STDATOMIC_H
- #include "ovs-atomic-c11.h"
#elif __has_extension(c_atomic)
#include "ovs-atomic-clang.h"
+ #elif HAVE_STDATOMIC_H
+ #include "ovs-atomic-c11.h"
#elif __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
#include "ovs-atomic-gcc4.7+.h"
+ #elif __GNUC__ && defined(__x86_64__)
+ #include "ovs-atomic-x86_64.h"
+ #elif __GNUC__ && defined(__i386__)
+ #include "ovs-atomic-i586.h"
#elif HAVE_GCC4_ATOMICS
#include "ovs-atomic-gcc4+.h"
+ #elif _MSC_VER && _M_IX86 >= 500
+ #include "ovs-atomic-msvc.h"
#else
+ /* ovs-atomic-pthreads implementation is provided for portability.
+ * It might be too slow for real use because Open vSwitch is
+ * optimized for platforms where real atomic ops are available. */
#include "ovs-atomic-pthreads.h"
#endif
#undef IN_OVS_ATOMIC_H
typedef ATOMIC(uint8_t) atomic_uint8_t;
typedef ATOMIC(uint16_t) atomic_uint16_t;
typedef ATOMIC(uint32_t) atomic_uint32_t;
-typedef ATOMIC(uint64_t) atomic_uint64_t;
typedef ATOMIC(int8_t) atomic_int8_t;
typedef ATOMIC(int16_t) atomic_int16_t;
typedef ATOMIC(int32_t) atomic_int32_t;
-typedef ATOMIC(int64_t) atomic_int64_t;
+
+/* Relaxed atomic operations.
+ *
+ * When an operation on an atomic variable is not expected to synchronize
+ * with operations on other (atomic or non-atomic) variables, no memory
+ * barriers are needed and the relaxed memory ordering can be used. These
+ * macros make such uses less daunting, but not invisible. */
+#define atomic_store_relaxed(VAR, VALUE) \
+ atomic_store_explicit(VAR, VALUE, memory_order_relaxed)
+#define atomic_read_relaxed(VAR, DST) \
+ atomic_read_explicit(VAR, DST, memory_order_relaxed)
+#define atomic_compare_exchange_strong_relaxed(DST, EXP, SRC) \
+ atomic_compare_exchange_strong_explicit(DST, EXP, SRC, \
+ memory_order_relaxed, \
+ memory_order_relaxed)
+#define atomic_compare_exchange_weak_relaxed(DST, EXP, SRC) \
+ atomic_compare_exchange_weak_explicit(DST, EXP, SRC, \
+ memory_order_relaxed, \
+ memory_order_relaxed)
+#define atomic_add_relaxed(RMW, ARG, ORIG) \
+ atomic_add_explicit(RMW, ARG, ORIG, memory_order_relaxed)
+#define atomic_sub_relaxed(RMW, ARG, ORIG) \
+ atomic_sub_explicit(RMW, ARG, ORIG, memory_order_relaxed)
+#define atomic_or_relaxed(RMW, ARG, ORIG) \
+ atomic_or_explicit(RMW, ARG, ORIG, memory_order_relaxed)
+#define atomic_xor_relaxed(RMW, ARG, ORIG) \
+ atomic_xor_explicit(RMW, ARG, ORIG, memory_order_relaxed)
+#define atomic_and_relaxed(RMW, ARG, ORIG) \
+ atomic_and_explicit(RMW, ARG, ORIG, memory_order_relaxed)
+#define atomic_flag_test_and_set_relaxed(FLAG) \
+ atomic_flag_test_and_set_explicit(FLAG, memory_order_relaxed)
+#define atomic_flag_clear_relaxed(FLAG) \
+ atomic_flag_clear_explicit(FLAG, memory_order_relaxed)
+
+/* A simplified atomic count. Does not provide any synchronization with any
+ * other variables.
+ *
+ * Typically a counter is not used to synchronize the state of any other
+ * variables (with the notable exception of reference count, below).
+ * This abstraction releaves the user from the memory order considerations,
+ * and may make the code easier to read.
+ *
+ * We only support the unsigned int counters, as those are the most common. */
+typedef struct atomic_count {
+ atomic_uint count;
+} atomic_count;
+
+#define ATOMIC_COUNT_INIT(VALUE) { VALUE }
+
+static inline void
+atomic_count_init(atomic_count *count, unsigned int value)
+{
+ atomic_init(&count->count, value);
+}
+
+static inline unsigned int
+atomic_count_inc(atomic_count *count)
+{
+ unsigned int old;
+
+ atomic_add_relaxed(&count->count, 1, &old);
+
+ return old;
+}
+
+static inline unsigned int
+atomic_count_dec(atomic_count *count)
+{
+ unsigned int old;
+
+ atomic_sub_relaxed(&count->count, 1, &old);
+
+ return old;
+}
+
+static inline unsigned int
+atomic_count_get(atomic_count *count)
+{
+ unsigned int value;
+
+ atomic_read_relaxed(&count->count, &value);
+
+ return value;
+}
+
+static inline void
+atomic_count_set(atomic_count *count, unsigned int value)
+{
+ atomic_store_relaxed(&count->count, value);
+}
/* Reference count. */
struct ovs_refcount {
atomic_init(&refcount->count, 1);
}
-/* Destroys 'refcount'. */
-static inline void
-ovs_refcount_destroy(struct ovs_refcount *refcount)
-{
- atomic_destroy(&refcount->count);
-}
-
-/* Increments 'refcount'. */
+/* Increments 'refcount'.
+ *
+ * Does not provide a memory barrier, as the calling thread must have
+ * protected access to the object already. */
static inline void
ovs_refcount_ref(struct ovs_refcount *refcount)
{
unsigned int old_refcount;
- atomic_add(&refcount->count, 1, &old_refcount);
+ atomic_add_explicit(&refcount->count, 1, &old_refcount,
+ memory_order_relaxed);
ovs_assert(old_refcount > 0);
}
* // ...uninitialize object...
* free(object);
* }
- */
+ *
+ * Provides a release barrier making the preceding loads and stores to not be
+ * reordered after the unref, and in case of the last reference provides also
+ * an acquire barrier to keep all the following uninitialization from being
+ * reordered before the atomic decrement operation. Together these synchronize
+ * any concurrent unref operations between each other. */
static inline unsigned int
ovs_refcount_unref(struct ovs_refcount *refcount)
{
unsigned int old_refcount;
- atomic_sub(&refcount->count, 1, &old_refcount);
+ atomic_sub_explicit(&refcount->count, 1, &old_refcount,
+ memory_order_release);
ovs_assert(old_refcount > 0);
+ if (old_refcount == 1) {
+ /* 'memory_order_release' above means that there are no (reordered)
+ * accesses to the protected object from any thread at this point.
+ * An acquire barrier is needed to keep all subsequent access to the
+ * object's memory from being reordered before the atomic operation
+ * above. */
+ atomic_thread_fence(memory_order_acquire);
+ }
return old_refcount;
}
-/* Reads and returns 'ref_count_''s current reference count.
+/* Reads and returns 'refcount_''s current reference count.
+ *
+ * Does not provide a memory barrier.
*
* Rarely useful. */
static inline unsigned int
= CONST_CAST(struct ovs_refcount *, refcount_);
unsigned int count;
- atomic_read(&refcount->count, &count);
+ atomic_read_explicit(&refcount->count, &count, memory_order_relaxed);
return count;
}
+/* Increments 'refcount', but only if it is non-zero.
+ *
+ * This may only be called for an object which is RCU protected during
+ * this call. This implies that its possible destruction is postponed
+ * until all current RCU threads quiesce.
+ *
+ * Returns false if the refcount was zero. In this case the object may
+ * be safely accessed until the current thread quiesces, but no additional
+ * references to the object may be taken.
+ *
+ * Does not provide a memory barrier, as the calling thread must have
+ * RCU protected access to the object already.
+ *
+ * It is critical that we never increment a zero refcount to a
+ * non-zero value, as whenever a refcount reaches the zero value, the
+ * protected object may be irrevocably scheduled for deletion. */
+static inline bool
+ovs_refcount_try_ref_rcu(struct ovs_refcount *refcount)
+{
+ unsigned int count;
+
+ atomic_read_explicit(&refcount->count, &count, memory_order_relaxed);
+ do {
+ if (count == 0) {
+ return false;
+ }
+ } while (!atomic_compare_exchange_weak_explicit(&refcount->count, &count,
+ count + 1,
+ memory_order_relaxed,
+ memory_order_relaxed));
+ return true;
+}
+
+/* Decrements 'refcount' and returns the previous reference count. To
+ * be used only when a memory barrier is already provided for the
+ * protected object independently.
+ *
+ * For example:
+ *
+ * if (ovs_refcount_unref_relaxed(&object->ref_cnt) == 1) {
+ * // Schedule uninitialization and freeing of the object:
+ * ovsrcu_postpone(destructor_function, object);
+ * }
+ *
+ * Here RCU quiescing already provides a full memory barrier. No additional
+ * barriers are needed here.
+ *
+ * Or:
+ *
+ * if (stp && ovs_refcount_unref_relaxed(&stp->ref_cnt) == 1) {
+ * ovs_mutex_lock(&mutex);
+ * list_remove(&stp->node);
+ * ovs_mutex_unlock(&mutex);
+ * free(stp->name);
+ * free(stp);
+ * }
+ *
+ * Here a mutex is used to guard access to all of 'stp' apart from
+ * 'ref_cnt'. Hence all changes to 'stp' by other threads must be
+ * visible when we get the mutex, and no access after the unlock can
+ * be reordered to happen prior the lock operation. No additional
+ * barriers are needed here.
+ */
+static inline unsigned int
+ovs_refcount_unref_relaxed(struct ovs_refcount *refcount)
+{
+ unsigned int old_refcount;
+
+ atomic_sub_explicit(&refcount->count, 1, &old_refcount,
+ memory_order_relaxed);
+ ovs_assert(old_refcount > 0);
+ return old_refcount;
+}
+
#endif /* ovs-atomic.h */
projects / cascardo / ovs.git / blobdiff