Hi,
Михаил Бахтерев <m...@k.imm.uran.ru> writes:
> 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 letting us know the results of your testing, this is very
helpful. I just pushed a similar fix (same code, improved comments) to
the stable-2.2 branch, commit 2d09e0513fc11e2305077ba3653f6e4c2f266ddb,
which will be in the upcoming guile-2.2.5 release.
Thanks,
Mark
>
> 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 <m...@netris.org>
>> 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
>>
