Re: [PATCH] refcount_t: add ACQUIRE ordering on success for dec(sub)_and_test variants

[Dmitry Vyukov]

On Mon, Jan 28, 2019 at 1:10 PM Elena Reshetova
<elena.reshetova@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> ---
>  Documentation/core-api/refcount-vs-atomic.rst | 28 +++++++++++++++++++++++----
>  arch/x86/include/asm/refcount.h               | 21 ++++++++++++++++----
>  lib/refcount.c                                | 16 ++++++++++-----
>  3 files changed, 52 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/core-api/refcount-vs-atomic.rst b/Documentation/core-api/refcount-vs-atomic.rst
> index 322851b..95d4b4e 100644
> --- a/Documentation/core-api/refcount-vs-atomic.rst
> +++ b/Documentation/core-api/refcount-vs-atomic.rst
> @@ -54,6 +54,14 @@ 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 and all propagated stores from other CPUs
> +must propagate to all other CPUs after the acquire operation
> +(A-cumulative property). This is implemented using
> +:c:func:`smp_acquire__after_ctrl_dep`.

The second part starting from "It also guarantees that". I am not sure
I understand what it means. Is it just a copy-paste from RELEASE? I am
not sure ACQUIRE provides anything like this.


> +
>  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),
> @@ -119,24 +127,36 @@ 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 and control dependency
> +   on success.

Is ACQUIRE strictly stronger than control dependency?
It generally looks so unless there is something very subtle that I am
missing. If so, should we replace it with just "RELEASE ordering +
ACQUIRE ordering on success"? Looks simpler with less magic trickery.


> +
> +
> +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)``
>
>  Memory ordering guarantees changes:
>
> - * fully ordered --> RELEASE ordering + control dependency
> + * fully ordered --> RELEASE ordering + control dependency
>
>  .. 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 dbaed55..ab8f584 100644
> --- a/arch/x86/include/asm/refcount.h
> +++ b/arch/x86/include/asm/refcount.h
> @@ -67,16 +67,29 @@ 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 ebcf8cd..732feac 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 +167,7 @@ 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 +192,12 @@ 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 +209,7 @@ 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
>   */
> --
> 2.7.4
>