refcount_t: Add ACQUIRE ordering on success for dec(sub)_and_test() variants (47b8f3ab) · Commits · e / devices / android_kernel_fairphone_FP5

Documentation/core-api/refcount-vs-atomic.rst

+21 −3

Original line number	Diff line number	Diff line
		@@ -54,6 +54,13 @@ must propagate to all other CPUs before the release operation
		(A-cumulative property). This is implemented using
		:c:func:`smp_store_release`.

		An ACQUIRE memory ordering guarantees that all post loads and
		stores (all po-later instructions) on the same CPU are
		completed after the acquire operation. It also guarantees that all
		po-later stores on the same CPU must propagate to all other CPUs
		after the acquire operation executes. This is implemented using
		:c:func:`smp_acquire__after_ctrl_dep`.

		A control dependency (on success) for refcounters guarantees that
		if a reference for an object was successfully obtained (reference
		counter increment or addition happened, function returned true),
		@@ -119,13 +126,24 @@ Memory ordering guarantees changes:
		result of obtaining pointer to the object!


		case 5) - decrement-based RMW ops that return a value
		-----------------------------------------------------
		case 5) - generic dec/sub decrement-based RMW ops that return a value
		---------------------------------------------------------------------

		Function changes:

		* :c:func:`atomic_dec_and_test` --> :c:func:`refcount_dec_and_test`
		* :c:func:`atomic_sub_and_test` --> :c:func:`refcount_sub_and_test`

		Memory ordering guarantees changes:

		* fully ordered --> RELEASE ordering + ACQUIRE ordering on success


		case 6) other decrement-based RMW ops that return a value
		---------------------------------------------------------

		Function changes:

		* no atomic counterpart --> :c:func:`refcount_dec_if_one`
		* ``atomic_add_unless(&var, -1, 1)`` --> ``refcount_dec_not_one(&var)``

		@@ -136,7 +154,7 @@ Memory ordering guarantees changes:
		.. note:: :c:func:`atomic_add_unless` only provides full order on success.


		case 6) - lock-based RMW
		case 7) - lock-based RMW
		------------------------

		Function changes:

arch/x86/include/asm/refcount.h

+18 −4

Original line number	Diff line number	Diff line
		@@ -67,16 +67,30 @@ static __always_inline void refcount_dec(refcount_t *r)
		static __always_inline __must_check
		bool refcount_sub_and_test(unsigned int i, refcount_t *r)
		{
		return GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl",
		bool ret = GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl",
		REFCOUNT_CHECK_LT_ZERO,
		r->refs.counter, e, "er", i, "cx");

		if (ret) {
		smp_acquire__after_ctrl_dep();
		return true;
		}

		return false;
		}

		static __always_inline __must_check bool refcount_dec_and_test(refcount_t *r)
		{
		return GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl",
		bool ret = GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl",
		REFCOUNT_CHECK_LT_ZERO,
		r->refs.counter, e, "cx");

		if (ret) {
		smp_acquire__after_ctrl_dep();
		return true;
		}

		return false;
		}

		static __always_inline __must_check

lib/refcount.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -33,6 +33,9 @@
		* Note that the allocator is responsible for ordering things between free()
		* and alloc().
		*
		* The decrements dec_and_test() and sub_and_test() also provide acquire
		* ordering on success.
		*
		*/

		#include <linux/mutex.h>
		@@ -164,8 +167,8 @@ EXPORT_SYMBOL(refcount_inc_checked);
		* at UINT_MAX.
		*
		* Provides release memory ordering, such that prior loads and stores are done
		* before, and provides a control dependency such that free() must come after.
		* See the comment on top.
		* before, and provides an acquire ordering on success such that free()
		* must come after.
		*
		* Use of this function is not recommended for the normal reference counting
		* use case in which references are taken and released one at a time. In these
		@@ -190,7 +193,12 @@ bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r)

		} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));

		return !new;
		if (!new) {
		smp_acquire__after_ctrl_dep();
		return true;
		}
		return false;

		}
		EXPORT_SYMBOL(refcount_sub_and_test_checked);

		@@ -202,8 +210,8 @@ EXPORT_SYMBOL(refcount_sub_and_test_checked);
		* decrement when saturated at UINT_MAX.
		*
		* Provides release memory ordering, such that prior loads and stores are done
		* before, and provides a control dependency such that free() must come after.
		* See the comment on top.
		* before, and provides an acquire ordering on success such that free()
		* must come after.
		*
		* Return: true if the resulting refcount is 0, false otherwise
		*/