#147 QXK: PendSV_error is triggered on special conditions

[Anonymous]

QPC / v5.7.4

1. Active object with priority 34 is running
2. An interrupt triggers (having interrupt id=26)
3. QXK_sched() is run, in whitch:
   • QXK_attr_.curr == (void *)0 -> current thread is a basic thread
   • next->thread != 0 "this is an extened-thread"
     -> "no activation needed"
     -> QXK_attr_.next = next
     -> context switch
4. at the same time of QXK_sched processing, the same interrupt has been already triggered again (id=26), so instead of running PendSV, the interrupt handler is run again
5. this time interrupt handler (id=26) posts an event to a lower priority AO (priority=33)
6. QXK_sched() is run, in whitch:
   • QXK_attr_.curr == (void *)0 -> current thread is a basic thread
   • next->thread == (void *)0 -> next is a basic thread
   • if (p <= QXK_attr_.actPrio) { -> the priority is lower than current priority (33 vs 34)
   • QXK_attr_.next = (void )0, p= 0 / no activation needed */
7. PendSv is triggered
   -> because QXK_attr_.next is now 0, the precondition fails and PendSV_error is triggered

[Anonymous]

Here's a timeline capture what is happening.

[J. K. Paasikivi]

More info:
At step (3) above, the
QXK_attr.readySet.bits[0] = 0x10
QXK_attr.readySet.bits[1] = 0x00

At step (6) above, the
QXK_attr.readySet.bits[0] = 0x10
QXK_attr.readySet.bits[1] = 0x01

So the QXK_attr.readySet.bits[1] & 0x02 (for AO with priority 34) has been already cleared before this.
According to code, the only place to clear these seems to be QACTIVE_EQUEUE_ONEMPTY_() via QActive_get_() via QXK_activate_().

So in summary:

• Inside QXK_activate(), the interrupts are enabled QF_INT_ENABLE(); / unconditionally enable interrupts /
• QActive_get_() gets called -> (readySet.bits[1] & 0x02) gets zeroed
• interrupt (id=26) happens once, no posting of events this time
• interrupt (id=26) happens second time, this time posting an event to AO with priority of 33
• pendSV gets run -> PendSV_error

[Quantum Leaps]

Thank you for the information. The bug is now accepted and was assigned the highest priority. It will be fixed in the next QP/C/C++ release.

--MMS

[Quantum Leaps]

This bug has been fixed in QP/C 5.8.0 and QP/C++ 5.8.0.