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Squashed commit of the following: 

Author: patacongo <spudarnia@yahoo.com>

    Todo (#34)

    * Documentation/NuttXCCodingStandard.html:  Remove requirement to decorate ignored returned values with (void).

    * TODO:  Update TODO list

    Co-authored-by: Gregory Nutt <gnutt@nuttx.org>
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Gregory Nutt 2020-01-03 12:36:24 -03:00 committed by Alan Carvalho de Assis
parent d612fd3dc5
commit ebce5fb7ca
1 changed files with 49 additions and 23 deletions
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@ -1,4 +1,4 @@
NuttX TODO List (Last updated November 21, 2019)
NuttX TODO List (Last updated January 3, 2019)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This file summarizes known NuttX bugs, limitations, inconsistencies with
@ -589,32 +589,58 @@ o SMP
can that occur? I think it can occur in the following
situation:
CPU0 - Task A is running.
- The CPU0 IDLE task is the only other task in the
CPU0 ready-to-run list.
CPU1 - Task B is running.
- Task C is blocked but remains in the g_assignedtasks[]
list because of a CPU affinity selection. Task C
also holds the critical section which is temporarily
relinquished because Task C is blocked by Task B.
- The CPU1 IDLE task is at the end of the list.
The log below was reported is Nuttx running on two cores
Cortex-A7 architecture in SMP mode. You can notice see that
when sched_addreadytorun() was called, the g_cpu_irqset is 3.
Actions:
1. Task A/CPU 0 takes the critical section.
2. Task B/CPU 1 suspends waiting for an event
3. Task C is restarted.
sched_addreadytorun: irqset cpu 1, me 0 btcbname init, irqset 1 irqcount 2.
sched_addreadytorun: sched_addreadytorun line 338 g_cpu_irqset = 3.
Now both Task A and Task C hold the critical section.
This can happen, but only under a very certain condition.
g_cpu_irqset only exists to support this certain condition:
This problem has never been observed, but seems to be a
possibility. I believe it could only occur if CPU affinity
is used (otherwise, tasks will pend must as when pre-
emption is disabled).
a. A task running on CPU 0 takes the critical section. So
g_cpu_irqset == 0x1.
A proper solution would probably involve re-designing how
CPU affinity is implemented. The CPU1 IDLE thread should
more appropriately run, but cannot because the Task C TCB
is in the g_assignedtasks[] list.
b. A task exits on CPU 1 and a waiting, ready-to-run task
is re-started on CPU 1. This new task also holds the
critical section. So when the task is re-restarted on
CPU 1, we than have g_cpu_irqset == 0x3
So we are in a very perverse state! There are two tasks
running on two different CPUs and both hold the critical
section. I believe that is a dangerous situation and there
could be undiscovered bugs that could happen in that case.
However, as of this moment, I have not heard of any specific
problems caused by this weird behavior.
A possible solution would be to add a new task state that
would exist only for SMP.
- Add a new SMP-only task list and state. Say,
g_csection_wait[]. It should be prioritized.
- When a task acquires the critical section, all tasks in
g_readytorun[] that need the critical section would be
moved to g_csection_wait[].
- When any task is unblocked for any reason and moved to the
g_readytorun[] list, if that unblocked task needs the
critical section, it would also be moved to the
g_csection_wait[] list. No task that needs the critical
section can be in the ready-to-run list if the critical
section is not available.
- When the task releases the critical section, all tasks in
the g_csection_wait[] needs to be moved back to
g_readytorun[].
- This may result in a context switch. The tasks should be
moved back to g_readytorun[] higest priority first. If a
context switch occurs and the critical section to re-taken
by the re-started task, the lower priority tasks in
g_csection_wait[] must stay in that list.
That is really not as much work as it sounds. It is
something that could be done in 2-3 days of work if you know
what you are doing. Getting the proper test setup and
verifying the cahnge would be the more difficult task.
Status: Open
Priority: Unknown. Might be high, but first we would need to confirm