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masayuki pushed a commit to branch releases/10.0
in repository https://gitbox.apache.org/repos/asf/incubator-nuttx.git
commit f11418934b0884235c50b7dc649585e336a0933d
Author: Masayuki Ishikawa <masayuki.ishik...@gmail.com>
AuthorDate: Fri Nov 20 14:27:54 2020 +0900
Update TODO regarding SMP
Summary:
- 'POSSIBLE FOR TWO CPUs TO HOLD A CRITICAL SECTION' was resolved
Signed-off-by: Masayuki Ishikawa <masayuki.ishik...@jp.sony.com>
---
TODO | 66 +-----------------------------------------------------------------
1 file changed, 1 insertion(+), 65 deletions(-)
diff --git a/TODO b/TODO
index bffa549..d56cf2d 100644
--- a/TODO
+++ b/TODO
@@ -10,7 +10,7 @@ issues related to each board port.
nuttx/:
(16) Task/Scheduler (sched/)
- (3) SMP
+ (2) SMP
(1) Memory Management (mm/)
(0) Power Management (drivers/pm)
(5) Signals (sched/signal, arch/)
@@ -485,70 +485,6 @@ o SMP
an bugs caused by this. But I believe that failures are
possible.
- Title: POSSIBLE FOR TWO CPUs TO HOLD A CRITICAL SECTION?
- Description: The SMP design includes logic that will support multiple
- CPUs holding a critical section. Is this necessary? How
- can that occur? I think it can occur in the following
- situation:
-
- The log below was reported is Nuttx running on two cores
- Cortex-A7 architecture in SMP mode. You can notice see that
- when nxsched_add_readytorun() was called, the g_cpu_irqset is 3.
-
- nxsched_add_readytorun: irqset cpu 1, me 0 btcbname init,
irqset 1 irqcount 2.
- nxsched_add_readytorun: nxsched_add_readytorun line 338
g_cpu_irqset = 3.
-
- This can happen, but only under a very certain condition.
- g_cpu_irqset only exists to support this certain condition:
-
- a. A task running on CPU 0 takes the critical section. So
- g_cpu_irqset == 0x1.
-
- 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[] highest 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 change would be the more difficult task.
-
-Status: Open
-Priority: Unknown. Might be high, but first we would need to confirm
- that this situation can occur and that is actually causes
- a failure.
-
o Memory Management (mm/)
^^^^^^^^^^^^^^^^^^^^^^^
