Re: Rép. : Re: [Xenomai-help] Switch mode with x86

[Jan Kiszka <jan.kiszka@domain.hid>]

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Jan Kiszka wrote:
> ...
> This indicates that we face an I-pipe bug: the scheduled Linux call on
> relaxation of TASK2 and then later TASK1 somehow gets lost (there is no
> rthal_apc_handler in the remaining trace).

I think I got it. No I-pipe bug, but one in the HAL.

What happened? A weird race caused by the unprotected optimisation to
only call rthal_schedule_irq() if there is no APC pending yet. This is
the constellation I finally worked out via instrumenting and tracing:

PRIO 1:
rthal_apc_schedule()
  test&set rthal_apc_pending
  (but no rthal_schedule_irq() yet)

                        -PREEMPTION-

                                        PRIO 99:
                                        ...
                                        rthal_apc_schedule()
                                          test rthal_apc_pending
                                          (already set => no
                                          rthal_schedule_irq()!)

So, no one reported the ACP to I-pipe, and no one ever will in Nicolas
scenario - soft lock-up!

Nicolas, please give the attached patch a try. Your test is running fine
for me now.


At this chance: do we need rthal_apc_schedule() returning the previous
state at all? No current caller checks the return value. If it's OK to
clean this up, I will post a combined patch.

Jan

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Index: ksrc/arch/generic/hal.c
===================================================================
--- ksrc/arch/generic/hal.c	(Revision 1918)
+++ ksrc/arch/generic/hal.c	(Arbeitskopie)
@@ -596,18 +596,19 @@ int rthal_apc_free(int apc)
 int rthal_apc_schedule(int apc)
 {
     rthal_declare_cpuid;
+    int ret = 1;
 
     if (apc < 0 || apc >= RTHAL_NR_APCS)
         return -EINVAL;
 
     rthal_load_cpuid();         /* Migration would be harmless here. */
 
-    if (!test_and_set_bit(apc, &rthal_apc_pending[cpuid])) {
-        rthal_schedule_irq(rthal_apc_virq);
-        return 1;
-    }
+    if (test_and_set_bit(apc, &rthal_apc_pending[cpuid]))
+        ret = 0;                /* Already pending. */
+
+    rthal_schedule_irq(rthal_apc_virq);
 
-    return 0;                   /* Already pending. */
+    return ret;
 }
 
 #ifdef CONFIG_PROC_FS

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