From: Elena Reshetova elena.reshetova@intel.com
mainline inclusion from mainline-v5.1-rc1 commit 47b8f3ab9c49daa824af848f9e02889662d8638f category: bugfix bugzilla: NA CVE: NA
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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) peterz@infradead.org Signed-off-by: Elena Reshetova elena.reshetova@intel.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Andrea Parri andrea.parri@amarulasolutions.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Paul E. McKenney paulmck@linux.vnet.ibm.com Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Will Deacon will.deacon@arm.com Cc: dvyukov@google.com Cc: keescook@chromium.org Cc: stern@rowland.harvard.edu Link: https://lkml.kernel.org/r/1548847131-27854-2-git-send-email-elena.reshetova@... Signed-off-by: Ingo Molnar mingo@kernel.org Conflicts: arch/x86/include/asm/refcount.h [yyl: adjust context] Signed-off-by: Yang Yingliang yangyingliang@huawei.com Reviewed-by: Hanjun Guo guohanjun@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- Documentation/core-api/refcount-vs-atomic.rst | 24 ++++++++++++++++--- arch/x86/include/asm/refcount.h | 18 ++++++++++++-- lib/refcount.c | 18 ++++++++++---- 3 files changed, 50 insertions(+), 10 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 19b90521954c..88061df7add6 100644 --- a/arch/x86/include/asm/refcount.h +++ b/arch/x86/include/asm/refcount.h @@ -67,14 +67,28 @@ 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) { - GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl", REFCOUNT_CHECK_LT_ZERO, + bool ret = GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl", REFCOUNT_CHECK_LT_ZERO, r->refs.counter, "er", i, "%0", e, "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) { - GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl", REFCOUNT_CHECK_LT_ZERO, + bool ret = GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl", REFCOUNT_CHECK_LT_ZERO, r->refs.counter, "%0", 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 */