Yes, I understood that, what if CPU A and CPU B enters the unlock routine with
1 lockcount on the same task and after the decrement they both go into
preemption and manipulating with linked list task list?
On April 24, 2025 8:06:36 AM UTC, hujun260 <hujun...@163.com> wrote:
>The lockcount can be understood as a local variable, so no race conditions
>will occur.
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>At 2025-04-24 15:54:48, "Serg Podtynnyi" <s...@podtynnyi.com.INVALID> wrote:
>>Hi, I tried my best to understand the logic behind unlock and
>>merge_pending routines and come up with this fix that works for me.
>>I split the lockcount handling in 2 parts when lockcount is > 1 just
>>decrement fast as always,
>>but when it's 1 we enter critical section and make it 0 inside
>>it(pre-emption is enabled).
>>I think there is a small time after decrement in the if statement and
>> enter_critical_section_wo_note() in original code where can race happen.
>>Does this make sense? Sorry if it's a dumb question - scheduler stuff
>>with SMP is rocket-science level for me yet.
>>
>>diff --git a/sched/sched/sched_unlock.c b/sched/sched/sched_unlock.c
>>
>>index 4c7d681454..15fc39b546 100644
>>--- a/sched/sched/sched_unlock.c
>>+++ b/sched/sched/sched_unlock.c
>>@@ -65,13 +65,14 @@ void sched_unlock(void)
>>
>> DEBUGASSERT(rtcb == NULL || rtcb->lockcount > 0);
>>
>>- /* Check if the lock counter has decremented to zero. If so,
>>- * then pre-emption has been re-enabled.
>>- */
>>-
>>- if (rtcb != NULL && --rtcb->lockcount == 0)
>>+ if (rtcb != NULL && rtcb->lockcount > 1)
>>+ {
>>+ rtcb->lockcount--;
>>+ }
>>+ else if (rtcb != NULL && rtcb->lockcount == 1)
>> {
>> irqstate_t flags = enter_critical_section_wo_note();
>>+ rtcb->lockcount = 0;
>>
>> /* Note that we no longer have pre-emption disabled. */
>>
>>--
>>2.49.0
>>On 4/22/25 14:32, hujun260 wrote:
>>
>>It seems to have nothing to do with the race condition, and since the rtcb is
>>this_task, it will only be modified by one CPU.<br/><br/>As for why your
>>modification can solve the problem, I haven't figured it out yet. Can you
>>further analyze the reasons for the failure to boot?
>>At 2025-04-22 13:25:13, "Serg Podtynnyi" <s...@podtynnyi.com.INVALID> wrote:
>>
>>> Hi All, Gregory, Sebastien,
>>>
>>> I'm porting NuttX on rp2350(pico2 board inside ClockWork PicoCalc) and
>>> having trouble getting SMP (2-core) config running
>>>
>>> without patching the sched lock/unlock routines.
>>>
>>> I am looking at the commit
>>> https://github.com/apache/nuttx/commit/914ae532e68bf4ca75c758d6f541a1d2fcdce8c3
>>> and reverting it helps to boot the system.
>>>
>>> This the the patch to make it work for me, I updated the current
>>> lock/unlock by moving the enter_critical_section above the access to
>>> rtcb->lockcount
>>>
>>> Do you think it could be the case of a race condition with rtcb->lockcount ?
>>>
>>> PS
>>>
>>> It's almost 11 years since I last touched NuttX - it's a please to work
>>> with it again.
>>>
>>>
>>> From 86fa0a65fdd37804154faf3db52e2826281cdfbd Mon Sep 17 00:00:00 2001
>>> From: Serg Podtynnyi <s...@podtynnyi.com>
>>> Date: Tue, 22 Apr 2025 11:37:48 +0700
>>> Subject: sched: update lock/unlock critical section entry
>>>
>>> Level up enter_critical_section in sched lock/unlock to cover
>>> rtcb->lockcount access
>>>
>>> Signed-off-by: Serg Podtynnyi <s...@podtynnyi.com>
>>> ---
>>> sched/sched/sched_lock.c | 4 ++--
>>> sched/sched/sched_unlock.c | 4 ++--
>>> 2 files changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/sched/sched/sched_lock.c b/sched/sched/sched_lock.c
>>> index d8f75e7c90..9cc8da3df4 100644
>>> --- a/sched/sched/sched_lock.c
>>> +++ b/sched/sched/sched_lock.c
>>> @@ -82,18 +82,18 @@ void sched_lock(void)
>>> * operations on this thread (on any CPU)
>>> */
>>>
>>> + irqstate_t flags = enter_critical_section_wo_note();
>>> if (rtcb != NULL && rtcb->lockcount++ == 0)
>>> {
>>> #if (CONFIG_SCHED_CRITMONITOR_MAXTIME_PREEMPTION >= 0) || \
>>> defined(CONFIG_SCHED_INSTRUMENTATION_PREEMPTION)
>>> - irqstate_t flags = enter_critical_section_wo_note();
>>>
>>> /* Note that we have pre-emption locked */
>>>
>>> nxsched_critmon_preemption(rtcb, true, return_address(0));
>>> sched_note_preemption(rtcb, true);
>>> - leave_critical_section_wo_note(flags);
>>> #endif
>>> }
>>> + leave_critical_section_wo_note(flags);
>>> }
>>> }
>>> diff --git a/sched/sched/sched_unlock.c b/sched/sched/sched_unlock.c
>>> index 4c7d681454..d21e007587 100644
>>> --- a/sched/sched/sched_unlock.c
>>> +++ b/sched/sched/sched_unlock.c
>>> @@ -69,9 +69,9 @@ void sched_unlock(void)
>>> * then pre-emption has been re-enabled.
>>> */
>>>
>>> + irqstate_t flags = enter_critical_section_wo_note();
>>> if (rtcb != NULL && --rtcb->lockcount == 0)
>>> {
>>> - irqstate_t flags = enter_critical_section_wo_note();
>>>
>>> /* Note that we no longer have pre-emption disabled. */
>>>
>>> @@ -162,7 +162,7 @@ void sched_unlock(void)
>>> }
>>> #endif
>>>
>>> - leave_critical_section_wo_note(flags);
>>> }
>>> + leave_critical_section_wo_note(flags);
>>> }
>>> }
>>> --
>>> 2.49.0
>>>
>>>
>>> --
>>> Serg Podtynnyi
>>>
>>-- Serg Podtynnyi
