Commit-ID: 47b8f3ab9c49daa824af848f9e02889662d8638f Gitweb: https://git.kernel.org/tip/47b8f3ab9c49daa824af848f9e02889662d8638f Author: Elena Reshetova <elena.reshet...@intel.com> AuthorDate: Wed, 30 Jan 2019 13:18:51 +0200 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 4 Feb 2019 09:03:31 +0100
refcount_t: Add ACQUIRE ordering on success for dec(sub)_and_test() variants This adds an smp_acquire__after_ctrl_dep() barrier on successful decrease of refcounter value from 1 to 0 for refcount_dec(sub)_and_test variants and therefore gives stronger memory ordering guarantees than prior versions of these functions. Co-developed-by: Peter Zijlstra (Intel) <pet...@infradead.org> Signed-off-by: Elena Reshetova <elena.reshet...@intel.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Reviewed-by: Andrea Parri <andrea.pa...@amarulasolutions.com> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Will Deacon <will.dea...@arm.com> Cc: dvyu...@google.com Cc: keesc...@chromium.org Cc: st...@rowland.harvard.edu Link: https://lkml.kernel.org/r/1548847131-27854-2-git-send-email-elena.reshet...@intel.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- Documentation/core-api/refcount-vs-atomic.rst | 24 +++++++++++++++++++++--- arch/x86/include/asm/refcount.h | 22 ++++++++++++++++++---- lib/refcount.c | 18 +++++++++++++----- 3 files changed, 52 insertions(+), 12 deletions(-) diff --git a/Documentation/core-api/refcount-vs-atomic.rst b/Documentation/core-api/refcount-vs-atomic.rst index 322851bada16..976e85adffe8 100644 --- a/Documentation/core-api/refcount-vs-atomic.rst +++ b/Documentation/core-api/refcount-vs-atomic.rst @@ -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: diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h index dbaed55c1c24..232f856e0db0 100644 --- a/arch/x86/include/asm/refcount.h +++ b/arch/x86/include/asm/refcount.h @@ -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", - REFCOUNT_CHECK_LT_ZERO, - r->refs.counter, e, "cx"); + 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 diff --git a/lib/refcount.c b/lib/refcount.c index ebcf8cd49e05..6e904af0fb3e 100644 --- a/lib/refcount.c +++ b/lib/refcount.c @@ -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 */
