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

[Andrea Parri]

On Wed, Jan 30, 2019 at 01:18:51PM +0200, Elena Reshetova 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>

Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>

  Andrea


> ---
>  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 322851b..976e85a 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 dbaed55..232f856 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 ebcf8cd..6e904af 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
>   */
> -- 
> 2.7.4
>