X-Git-Url: https://git.saurik.com/apple/xnu.git/blobdiff_plain/0a7de7458d150b5d4dffc935ba399be265ef0a1a..cb3231590a3c94ab4375e2228bd5e86b0cf1ad7e:/osfmk/machine/atomic.h

diff --git a/osfmk/machine/atomic.h b/osfmk/machine/atomic.h
index 3c3676248..ab11c7004 100644
--- a/osfmk/machine/atomic.h
+++ b/osfmk/machine/atomic.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015 Apple Inc. All rights reserved.
+ * Copyright (c) 2015-2018 Apple Inc. All rights reserved.
  *
  * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
  *
@@ -29,82 +29,735 @@
 #ifndef _MACHINE_ATOMIC_H
 #define _MACHINE_ATOMIC_H
 
-#include <stdatomic.h>
+/*
+ * Internal implementation details are in a separate header
+ */
+#include <machine/atomic_impl.h>
+
+/*!
+ * @file <machine/atomic.h>
+ *
+ * @brief
+ * This file defines nicer (terser and safer) wrappers for C11's <stdatomic.h>.
+ *
+ * @discussion
+ * @see xnu.git::doc/atomics.md which provides more extensive documentation
+ * about this header.
+ *
+ * Note that some of the macros defined in this file may be overridden by
+ * architecture specific headers.
+ *
+ * All the os_atomic* functions take an operation ordering argument that can be:
+ * - C11 memory orders: relaxed, acquire, release, acq_rel or seq_cst which
+ *   imply a memory fence on SMP machines, and always carry the matching
+ *   compiler barrier semantics.
+ *
+ * - the os_atomic-specific `dependency` memory ordering that is used to
+ *   document intent to a carry a data or address dependency.
+ *   See doc/atomics.md for more information.
+ *
+ * - a compiler barrier: compiler_acquire, compiler_release, compiler_acq_rel
+ *   without a corresponding memory fence.
+ */
+
+/*!
+ * @function os_compiler_barrier
+ *
+ * @brief
+ * Provide a compiler barrier according to the specified ordering.
+ *
+ * @param m
+ * An optional ordering among `acquire`, `release` or `acq_rel` which defaults
+ * to `acq_rel` when not specified.
+ * These are equivalent to the `compiler_acquire`, `compiler_release` and
+ * `compiler_acq_rel` orderings taken by the os_atomic* functions
+ */
+#define os_compiler_barrier(b...) \
+	        atomic_signal_fence(_os_compiler_barrier_##b)
+
+/*!
+ * @function os_atomic_thread_fence
+ *
+ * @brief
+ * Memory fence which is elided in non-SMP mode, but always carries the
+ * corresponding compiler barrier.
+ *
+ * @param m
+ * The ordering for this fence.
+ */
+#define os_atomic_thread_fence(m)  ({ \
+	        atomic_thread_fence(memory_order_##m##_smp); \
+	        atomic_signal_fence(memory_order_##m); \
+})
 
-#define _os_atomic_c11_atomic(p) \
-	        ((typeof(*(p)) _Atomic *)(p))
+/*!
+ * @function os_atomic_init
+ *
+ * @brief
+ * Wrapper for C11 atomic_init()
+ *
+ * @discussion
+ * This initialization is not performed atomically, and so must only be used as
+ * part of object initialization before the object is made visible to other
+ * threads/cores.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to initialize the variable with.
+ *
+ * @returns
+ * The value loaded from @a p.
+ */
+#define os_atomic_init(p, v) \
+	        atomic_init(_os_atomic_c11_atomic(p), v)
 
-#define _os_atomic_basetypeof(p) \
-	        typeof(atomic_load(((typeof(*(p)) _Atomic *)(p))))
+/*!
+ * @function os_atomic_load_is_plain, os_atomic_store_is_plain
+ *
+ * @brief
+ * Return whether a relaxed atomic load (resp. store) to an atomic variable
+ * is implemented as a single plain load (resp. store) instruction.
+ *
+ * @discussion
+ * Non-relaxed loads/stores may involve additional memory fence instructions
+ * or more complex atomic instructions.
+ *
+ * This is a construct that can safely be used in static asserts.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @returns
+ * True when relaxed atomic loads (resp. stores) compile to a plain load
+ * (resp. store) instruction, false otherwise.
+ */
+#define os_atomic_load_is_plain(p)  (sizeof(*(p)) <= sizeof(void *))
+#define os_atomic_store_is_plain(p) os_atomic_load_is_plain(p)
 
-#define _os_atomic_c11_op_orig(p, v, m, o) \
-	        atomic_##o##_explicit(_os_atomic_c11_atomic(p), v, \
-	        memory_order_##m)
+/*!
+ * @function os_atomic_load
+ *
+ * @brief
+ * Wrapper for C11 atomic_load_explicit(), guaranteed to compile to a single
+ * plain load instruction (when @a m is `relaxed`).
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * The value loaded from @a p.
+ */
+#define os_atomic_load(p, m)  ({ \
+	        _Static_assert(os_atomic_load_is_plain(p), "Load is wide"); \
+	        _os_atomic_basetypeof(p) _r; \
+	        _os_compiler_barrier_before_atomic(m); \
+	        _r = atomic_load_explicit(_os_atomic_c11_atomic(p), \
+	                        memory_order_##m##_smp); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        _r; \
+})
 
-#define _os_atomic_c11_op(p, v, m, o, op) \
-	        ({ typeof(v) _v = (v); _os_atomic_c11_op_orig(p, v, m, o) op _v; })
+/*!
+ * @function os_atomic_load_wide
+ *
+ * @brief
+ * Wrapper for C11 atomic_load_explicit(), which may be implemented by a
+ * compare-exchange loop for double-wide variables.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * The value loaded from @a p.
+ */
+#define os_atomic_load_wide(p, m)  ({ \
+	        _os_atomic_basetypeof(p) _r; \
+	        _os_compiler_barrier_before_atomic(m); \
+	        _r = atomic_load_explicit(_os_atomic_c11_atomic(p), \
+	                        memory_order_##m##_smp); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        _r; \
+})
 
-#define os_atomic_thread_fence(m)  atomic_thread_fence(memory_order_##m)
+/*!
+ * @function os_atomic_store
+ *
+ * @brief
+ * Wrapper for C11 atomic_store_explicit(), guaranteed to compile to a single
+ * plain store instruction (when @a m is `relaxed`).
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to store.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * The value stored at @a p.
+ */
+#define os_atomic_store(p, v, m)  ({ \
+	        _Static_assert(os_atomic_store_is_plain(p), "Store is wide"); \
+	        _os_atomic_basetypeof(p) _v = (v); \
+	        _os_compiler_barrier_before_atomic(m); \
+	        atomic_store_explicit(_os_atomic_c11_atomic(p), _v, \
+	                        memory_order_##m##_smp); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        _v; \
+})
 
-#define os_atomic_load(p, m) \
-	        atomic_load_explicit(_os_atomic_c11_atomic(p), memory_order_##m)
-#define os_atomic_store(p, v, m)    _os_atomic_c11_op_orig(p, v, m, store)
+/*!
+ * @function os_atomic_store_wide
+ *
+ * @brief
+ * Wrapper for C11 atomic_store_explicit(), which may be implemented by a
+ * compare-exchange loop for double-wide variables.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to store.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * The value stored at @a p.
+ */
+#define os_atomic_store_wide(p, v, m)  ({ \
+	        _os_atomic_basetypeof(p) _v = (v); \
+	        _os_compiler_barrier_before_atomic(m); \
+	        atomic_store_explicit(_os_atomic_c11_atomic(p), _v, \
+	                        memory_order_##m##_smp); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        _v; \
+})
 
+/*!
+ * @function os_atomic_add, os_atomic_add_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_add_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to add.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_add_orig returns the value of the variable before the atomic add,
+ * os_atomic_add returns the value of the variable after the atomic add.
+ */
 #define os_atomic_add_orig(p, v, m) _os_atomic_c11_op_orig(p, v, m, fetch_add)
 #define os_atomic_add(p, v, m)      _os_atomic_c11_op(p, v, m, fetch_add, +)
 
+/*!
+ * @function os_atomic_inc, os_atomic_inc_orig
+ *
+ * @brief
+ * Perform an atomic increment.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_inc_orig returns the value of the variable before the atomic increment,
+ * os_atomic_inc returns the value of the variable after the atomic increment.
+ */
 #define os_atomic_inc_orig(p, m)    _os_atomic_c11_op_orig(p, 1, m, fetch_add)
 #define os_atomic_inc(p, m)         _os_atomic_c11_op(p, 1, m, fetch_add, +)
 
+/*!
+ * @function os_atomic_sub, os_atomic_sub_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_sub_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to subtract.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_sub_orig returns the value of the variable before the atomic subtract,
+ * os_atomic_sub returns the value of the variable after the atomic subtract.
+ */
 #define os_atomic_sub_orig(p, v, m) _os_atomic_c11_op_orig(p, v, m, fetch_sub)
 #define os_atomic_sub(p, v, m)      _os_atomic_c11_op(p, v, m, fetch_sub, -)
 
+/*!
+ * @function os_atomic_dec, os_atomic_dec_orig
+ *
+ * @brief
+ * Perform an atomic decrement.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_dec_orig returns the value of the variable before the atomic decrement,
+ * os_atomic_dec returns the value of the variable after the atomic decrement.
+ */
 #define os_atomic_dec_orig(p, m)    _os_atomic_c11_op_orig(p, 1, m, fetch_sub)
 #define os_atomic_dec(p, m)         _os_atomic_c11_op(p, 1, m, fetch_sub, -)
 
+/*!
+ * @function os_atomic_and, os_atomic_and_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_and_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to and.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_and_orig returns the value of the variable before the atomic and,
+ * os_atomic_and returns the value of the variable after the atomic and.
+ */
 #define os_atomic_and_orig(p, v, m) _os_atomic_c11_op_orig(p, v, m, fetch_and)
 #define os_atomic_and(p, v, m)      _os_atomic_c11_op(p, v, m, fetch_and, &)
 
+/*!
+ * @function os_atomic_andnot, os_atomic_andnot_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_and_explicit(p, ~value).
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value whose complement to and.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_andnot_orig returns the value of the variable before the atomic andnot,
+ * os_atomic_andnot returns the value of the variable after the atomic andnot.
+ */
+#define os_atomic_andnot_orig(p, v, m) _os_atomic_c11_op_orig(p, ~(v), m, fetch_and)
+#define os_atomic_andnot(p, v, m)      _os_atomic_c11_op(p, ~(v), m, fetch_and, &)
+
+/*!
+ * @function os_atomic_or, os_atomic_or_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_or_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to or.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_or_orig returns the value of the variable before the atomic or,
+ * os_atomic_or returns the value of the variable after the atomic or.
+ */
 #define os_atomic_or_orig(p, v, m)  _os_atomic_c11_op_orig(p, v, m, fetch_or)
 #define os_atomic_or(p, v, m)       _os_atomic_c11_op(p, v, m, fetch_or, |)
 
+/*!
+ * @function os_atomic_xor, os_atomic_xor_orig
+ *
+ * @brief
+ * Wrappers for C11 atomic_fetch_xor_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to xor.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_xor_orig returns the value of the variable before the atomic xor,
+ * os_atomic_xor returns the value of the variable after the atomic xor.
+ */
 #define os_atomic_xor_orig(p, v, m) _os_atomic_c11_op_orig(p, v, m, fetch_xor)
 #define os_atomic_xor(p, v, m)      _os_atomic_c11_op(p, v, m, fetch_xor, ^)
 
+/*!
+ * @function os_atomic_min, os_atomic_min_orig
+ *
+ * @brief
+ * Wrappers for Clang's __atomic_fetch_min()
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to minimize.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_min_orig returns the value of the variable before the atomic min,
+ * os_atomic_min returns the value of the variable after the atomic min.
+ */
+#define os_atomic_min_orig(p, v, m) _os_atomic_clang_op_orig(p, v, m, fetch_min)
+#define os_atomic_min(p, v, m)      _os_atomic_clang_op(p, v, m, fetch_min, MIN)
+
+/*!
+ * @function os_atomic_max, os_atomic_max_orig
+ *
+ * @brief
+ * Wrappers for Clang's __atomic_fetch_max()
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to maximize.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * os_atomic_max_orig returns the value of the variable before the atomic max,
+ * os_atomic_max returns the value of the variable after the atomic max.
+ */
+#define os_atomic_max_orig(p, v, m) _os_atomic_clang_op_orig(p, v, m, fetch_max)
+#define os_atomic_max(p, v, m)      _os_atomic_clang_op(p, v, m, fetch_max, MAX)
+
+/*!
+ * @function os_atomic_xchg
+ *
+ * @brief
+ * Wrapper for C11 atomic_exchange_explicit().
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param v
+ * The value to exchange with.
+ *
+ * @param m
+ * The ordering to use.
+ *
+ * @returns
+ * The value of the variable before the exchange.
+ */
 #define os_atomic_xchg(p, v, m)     _os_atomic_c11_op_orig(p, v, m, exchange)
 
-#define os_atomic_cmpxchg(p, e, v, m) \
-	        ({ _os_atomic_basetypeof(p) _r = (e); \
-	        atomic_compare_exchange_strong_explicit(_os_atomic_c11_atomic(p), \
-	        &_r, v, memory_order_##m, memory_order_relaxed); })
-#define os_atomic_cmpxchgv(p, e, v, g, m) \
-	        ({ _os_atomic_basetypeof(p) _r = (e); int _b = \
-	        atomic_compare_exchange_strong_explicit(_os_atomic_c11_atomic(p), \
-	        &_r, v, memory_order_##m, memory_order_relaxed); *(g) = _r; _b; })
-#define os_atomic_cmpxchgvw(p, e, v, g, m) \
-	        ({ _os_atomic_basetypeof(p) _r = (e); int _b = \
-	        atomic_compare_exchange_weak_explicit(_os_atomic_c11_atomic(p), \
-	        &_r, v, memory_order_##m, memory_order_relaxed); *(g) = _r;  _b; })
+/*!
+ * @function os_atomic_cmpxchg
+ *
+ * @brief
+ * Wrapper for C11 atomic_compare_exchange_strong_explicit().
+ *
+ * @discussion
+ * Loops around os_atomic_cmpxchg() may want to consider using the
+ * os_atomic_rmw_loop() construct instead to take advantage of the C11 weak
+ * compare-exchange operation.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param e
+ * The value expected in the atomic variable.
+ *
+ * @param v
+ * The value to store if the atomic variable has the expected value @a e.
+ *
+ * @param m
+ * The ordering to use in case of success.
+ * The ordering in case of failure is always `relaxed`.
+ *
+ * @returns
+ * 0 if the compare-exchange failed.
+ * 1 if the compare-exchange succeeded.
+ */
+#define os_atomic_cmpxchg(p, e, v, m)  ({ \
+	        _os_atomic_basetypeof(p) _r = (e); int _b; \
+	        _os_compiler_barrier_before_atomic(m); \
+	        _b = atomic_compare_exchange_strong_explicit(_os_atomic_c11_atomic(p), \
+	                        &_r, v, memory_order_##m##_smp, memory_order_relaxed); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        _b; \
+})
+
+/*!
+ * @function os_atomic_cmpxchgv
+ *
+ * @brief
+ * Wrapper for C11 atomic_compare_exchange_strong_explicit().
+ *
+ * @discussion
+ * Loops around os_atomic_cmpxchgv() may want to consider using the
+ * os_atomic_rmw_loop() construct instead to take advantage of the C11 weak
+ * compare-exchange operation.
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param e
+ * The value expected in the atomic variable.
+ *
+ * @param v
+ * The value to store if the atomic variable has the expected value @a e.
+ *
+ * @param g
+ * A pointer to a location that is filled with the value that was present in
+ * the atomic variable before the compare-exchange (whether successful or not).
+ * This can be used to redrive compare-exchange loops.
+ *
+ * @param m
+ * The ordering to use in case of success.
+ * The ordering in case of failure is always `relaxed`.
+ *
+ * @returns
+ * 0 if the compare-exchange failed.
+ * 1 if the compare-exchange succeeded.
+ */
+#define os_atomic_cmpxchgv(p, e, v, g, m)  ({ \
+	        _os_atomic_basetypeof(p) _r = (e); int _b; \
+	        _os_compiler_barrier_before_atomic(m); \
+	        _b = atomic_compare_exchange_strong_explicit(_os_atomic_c11_atomic(p), \
+	                        &_r, v, memory_order_##m##_smp, memory_order_relaxed); \
+	        _os_compiler_barrier_after_atomic(m); \
+	        *(g) = _r; _b; \
+})
 
+/*!
+ * @function os_atomic_rmw_loop
+ *
+ * @brief
+ * Advanced read-modify-write construct to wrap compare-exchange loops.
+ *
+ * @param p
+ * A pointer to an atomic variable to be modified.
+ *
+ * @param ov
+ * The name of the variable that will contain the original value of the atomic
+ * variable (reloaded every iteration of the loop).
+ *
+ * @param nv
+ * The name of the variable that will contain the new value to compare-exchange
+ * the atomic variable to (typically computed from @a ov every iteration of the
+ * loop).
+ *
+ * @param m
+ * The ordering to use in case of success.
+ * The ordering in case of failure is always `relaxed`.
+ *
+ * @param ...
+ * Code block that validates the value of @p ov and computes the new value of
+ * @p nv that the atomic variable will be compare-exchanged to in an iteration
+ * of the loop.
+ *
+ * The loop can be aborted using os_atomic_rmw_loop_give_up(), e.g. when the
+ * value of @p ov is found to be "invalid" for the ovarall operation.
+ * `continue` cannot be used in this context.
+ *
+ * No stores to memory should be performed within the code block as it may cause
+ * LL/SC transactions used to implement compare-exchange to fail persistently.
+ *
+ * @returns
+ * 0 if the loop was aborted with os_atomic_rmw_loop_give_up().
+ * 1 if the loop completed.
+ */
 #define os_atomic_rmw_loop(p, ov, nv, m, ...)  ({ \
-	        bool _result = false; \
+	        int _result = 0; \
 	        typeof(p) _p = (p); \
-	        ov = os_atomic_load(_p, relaxed); \
+	        _os_compiler_barrier_before_atomic(m); \
+	        ov = atomic_load_explicit(_os_atomic_c11_atomic(_p), \
+	                        memory_order_relaxed); \
 	        do { \
 	                __VA_ARGS__; \
-	                _result = os_atomic_cmpxchgvw(_p, ov, nv, &ov, m); \
-	        } while (!_result); \
+	                _result = atomic_compare_exchange_weak_explicit( \
+	                                _os_atomic_c11_atomic(_p), &ov, nv, \
+	                                memory_order_##m##_smp, memory_order_relaxed); \
+	        } while (__builtin_expect(!_result, 0)); \
+	        _os_compiler_barrier_after_atomic(m); \
 	        _result; \
 	})
 
-#define os_atomic_rmw_loop_give_up_with_fence(m, expr) \
-	        ({ os_atomic_thread_fence(m); expr; __builtin_unreachable(); })
-#define os_atomic_rmw_loop_give_up(expr) \
-	        os_atomic_rmw_loop_give_up_with_fence(relaxed, expr)
+/*!
+ * @function os_atomic_rmw_loop_give_up
+ *
+ * @brief
+ * Abort an os_atomic_rmw_loop() loop.
+ *
+ * @param ...
+ * Optional code block to execute before the `break` out of the loop. May
+ * further alter the control flow (e.g. using `return`, `goto`, ...).
+ */
+#define os_atomic_rmw_loop_give_up(...) ({ __VA_ARGS__; break; })
+
+/*!
+ * @typedef os_atomic_dependency_t
+ *
+ * @brief
+ * Type for dependency tokens that can be derived from loads with dependency
+ * and injected into various expressions.
+ *
+ * @warning
+ * The implementation of atomic dependencies makes painstakingly sure that the
+ * compiler doesn't know that os_atomic_dependency_t::__opaque_zero is always 0.
+ *
+ * Users of os_atomic_dependency_t MUST NOT test its value (even with an
+ * assert), as doing so would allow the compiler to reason about the value and
+ * elide its use to inject hardware dependencies (thwarting the entire purpose
+ * of the construct).
+ */
+typedef struct { unsigned long __opaque_zero; } os_atomic_dependency_t;
+
+/*!
+ * @const OS_ATOMIC_DEPENDENCY_NONE
+ *
+ * @brief
+ * A value to pass to functions that can carry dependencies, to indicate that
+ * no dependency should be carried.
+ */
+#define OS_ATOMIC_DEPENDENCY_NONE \
+	        ((os_atomic_dependency_t){ 0UL })
+
+/*!
+ * @function os_atomic_make_dependency
+ *
+ * @brief
+ * Create a dependency token that can be injected into expressions to force a
+ * hardware dependency.
+ *
+ * @discussion
+ * This function is only useful for cases where the dependency needs to be used
+ * several times.
+ *
+ * os_atomic_load_with_dependency_on() and os_atomic_inject_dependency() are
+ * otherwise capable of automatically creating dependency tokens.
+ *
+ * @param v
+ * The result of:
+ * - an os_atomic_load(..., dependency),
+ * - an os_atomic_inject_dependency(),
+ * - an os_atomic_load_with_dependency_on().
+ *
+ * Note that due to implementation limitations, the type of @p v must be
+ * register-sized, if necessary an explicit cast is required.
+ *
+ * @returns
+ * An os_atomic_dependency_t token that can be used to prolongate dependency
+ * chains.
+ *
+ * The token value is always 0, but the compiler must never be able to reason
+ * about that fact (c.f. os_atomic_dependency_t)
+ */
+#define os_atomic_make_dependency(v) \
+	        ((void)(v), OS_ATOMIC_DEPENDENCY_NONE)
+
+/*!
+ * @function os_atomic_inject_dependency
+ *
+ * @brief
+ * Inject a hardware dependency resulting from a `dependency` load into a
+ * specified pointer.
+ *
+ * @param p
+ * A pointer to inject the dependency into.
+ *
+ * @param e
+ * - a dependency token returned from os_atomic_make_dependency(),
+ *
+ * - OS_ATOMIC_DEPENDENCY_NONE, which turns this operation into a no-op,
+ *
+ * - any value accepted by os_atomic_make_dependency().
+ *
+ * @returns
+ * A value equal to @a p but that prolongates the dependency chain rooted at
+ * @a e.
+ */
+#define os_atomic_inject_dependency(p, e) \
+	        ((typeof(*(p)) *)((p) + _os_atomic_auto_dependency(e).__opaque_zero))
 
-#define os_atomic_force_dependency_on(p, e) (p)
+/*!
+ * @function os_atomic_load_with_dependency_on
+ *
+ * @brief
+ * Load that prolongates the dependency chain rooted at `v`.
+ *
+ * @discussion
+ * This is shorthand for:
+ *
+ * <code>
+ *   os_atomic_load(os_atomic_inject_dependency(p, e), dependency)
+ * </code>
+ *
+ * @param p
+ * A pointer to an atomic variable.
+ *
+ * @param e
+ * - a dependency token returned from os_atomic_make_dependency(),
+ *
+ * - OS_ATOMIC_DEPENDENCY_NONE, which turns this operation into a no-op,
+ *
+ * - any value accepted by os_atomic_make_dependency().
+ *
+ * @returns
+ * The value loaded from @a p.
+ */
 #define os_atomic_load_with_dependency_on(p, e) \
-	        os_atomic_load(os_atomic_force_dependency_on(p, e), relaxed)
+	        os_atomic_load(os_atomic_inject_dependency(p, e), dependency)
+
+/*!
+ * @const OS_ATOMIC_HAS_LLSC
+ *
+ * @brief
+ * Whether the platform has LL/SC features.
+ *
+ * @discussion
+ * When set, the os_atomic_*_exclusive() macros are defined.
+ */
+#define OS_ATOMIC_HAS_LLSC  0
+
+/*!
+ * @const OS_ATOMIC_USE_LLSC
+ *
+ * @brief
+ * Whether os_atomic* use LL/SC internally.
+ *
+ * @discussion
+ * OS_ATOMIC_USE_LLSC implies OS_ATOMIC_HAS_LLSC.
+ */
+#define OS_ATOMIC_USE_LLSC  0
 
 #if defined (__x86_64__)
 #include "i386/atomic.h"