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

[Andrea Parri]

On Mon, Jan 28, 2019 at 02:09:37PM +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>

+ Alan, Dmitry; they might also deserve a Suggested-by:  ;-)

[...]


> +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).

Mmh, this property (A-cumulativity) isn't really associated to ACQUIREs
in the LKMM; I'd suggest to simply remove the last sentence.

[...]


> 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;

There appears to be some white-space damage (here and in other places);
checkpatch.pl should point these and other style problems out.

  Andrea


>  }
>  
>  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
>