Subject: Re: bug#32786: (system* ...) call somehow interferes with atomic-box on armv7l

From: Михаил Бахтерев <[email-masked]>
Date: Sat, 6 Oct 2018 00:43:57 +0500

Hi. I am sorry for delay.

I've tested the patch (i've applied it to the v2.2.4 source code). The
test sample works now as expected with both test «payloads»: (sleep
N-seconds) and (system* "any" "command" "here") calls.

Thanks for fixing it.

- MB. Respectfully

On Thu, Sep 27, 2018 at 01:49:23AM -0400, Mark H Weaver wrote:
> Hi,
> 
> Thanks for the additional details.  I was able to reproduce the bug, and
> I believe I now see the problem.
> 
> 'atomic-box-compare-and-swap!' is implemented using
> 'atomic_compare_exchange_weak' (if available), but neglects to handle
> the case where 'atomic_compare_exchange_weak' may spuriously fail.  In
> that case, the box is left unchanged, although the observed value was
> equal to the expected value.
> 
> What's happening here is that the 'atomic-box-compare-and-swap!' in
> 'sleep-loop' fails spuriously, leaving the box in state #:accepted
> although it returns #:accepted as its result.  When the main loop
> discovers this, it changes the state to #:need-to-sleep, although the
> thread has already ended.
> 
> To confirm this hypothesis, I added a print statement to the main loop
> showing the state of the box that it observed during the protocol
> exchange, and indeed it sees #:accepted the first time it checks, and
> #:need-to-sleep in all later iterations.
> 
> I've attached a proposed patch that I hope will fix this problem.  If
> you'd be willing to test it, I'd be grateful, but otherwise I'll try to
> test it in the next week or so.  My access to armv7l boxes is somewhat
> limited.
> 
> Thanks for this report.
> 
>       Mark
> 
> 

> From 958d37686a9ac65f48cb2ca32a341cf182c24b5a Mon Sep 17 00:00:00 2001
> From: Mark H Weaver <[email-masked]>
> Date: Thu, 27 Sep 2018 01:00:11 -0400
> Subject: [PATCH] UNTESTED: Fix 'atomic-box-compare-and-swap!'.
> 
> Fixes <https://bugs.gnu.org/32786>.
> 
> 'scm_atomic_compare_and_swap_scm' uses 'atomic_compare_exchange_weak' if
> it's available on the host platform.  'atomic_compare_exchange_weak' may
> fail spuriously, leaving the atomic object unchanged even when it
> contained the expected value.  'atomic-box-compare-and-swap!' uses
> 'scm_atomic_compare_and_swap_scm' without checking its return value, and
> therefore may return the expected value while leaving the box unchanged.
> This contradicts the documentation, which explicitly states that "you
> can know if the swap worked by checking if the return value is 'eq?' to
> EXPECTED.".
> 
> * libguile/atomic.c (scm_atomic_box_compare_and_swap_x): If
> 'scm_atomic_compare_and_swap_scm' returns false and the observed value
> is equal to 'expected', then try again.
> * libguile/vm-engine.c (atomic-box-compare-and-swap!): Ditto.
> ---
>  libguile/atomic.c    | 13 +++++++++----
>  libguile/vm-engine.c | 13 ++++++++-----
>  2 files changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/libguile/atomic.c b/libguile/atomic.c
> index 950874030..504643912 100644
> --- a/libguile/atomic.c
> +++ b/libguile/atomic.c
> @@ -1,4 +1,4 @@
> -/* Copyright (C) 2016 Free Software Foundation, Inc.
> +/* Copyright (C) 2016, 2018 Free Software Foundation, Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public License
> @@ -95,10 +95,15 @@ SCM_DEFINE (scm_atomic_box_compare_and_swap_x,
>              "if the return value is @code{eq?} to @var{expected}.")
>  #define FUNC_NAME s_scm_atomic_box_compare_and_swap_x
>  {
> +  SCM result = expected;
> +
>    SCM_VALIDATE_ATOMIC_BOX (1, box);
> -  scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (box),
> -                                   &expected, desired);
> -  return expected;
> +  while (!scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (box),
> +                                           &result, desired)
> +         && scm_is_eq (result, expected))
> +    {
> +    }
> +  return result;
>  }
>  #undef FUNC_NAME
>  
> diff --git a/libguile/vm-engine.c b/libguile/vm-engine.c
> index 19ff3e498..9650e33eb 100644
> --- a/libguile/vm-engine.c
> +++ b/libguile/vm-engine.c
> @@ -3868,16 +3868,19 @@ VM_NAME (scm_i_thread *thread, struct scm_vm *vp,
>      {
>        scm_t_uint16 dst, box;
>        scm_t_uint32 expected, desired;
> -      SCM scm_box, scm_expected;
> +      SCM scm_box, scm_expected, scm_result;
>        UNPACK_12_12 (op, dst, box);
>        UNPACK_24 (ip[1], expected);
>        UNPACK_24 (ip[2], desired);
>        scm_box = SP_REF (box);
>        VM_VALIDATE_ATOMIC_BOX (scm_box, "atomic-box-compare-and-swap!");
> -      scm_expected = SP_REF (expected);
> -      scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (scm_box),
> -                                       &scm_expected, SP_REF (desired));
> -      SP_SET (dst, scm_expected);
> +      scm_result = scm_expected = SP_REF (expected);
> +      while (!scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (scm_box),
> +                                               &scm_result, SP_REF (desired))
> +             && scm_is_eq (scm_result, scm_expected))
> +        {
> +        }
> +      SP_SET (dst, scm_result);
>        NEXT (3);
>      }
>  
> -- 
> 2.19.0
>