Jan Kiszka wrote:
Philippe Gerum wrote:

Jan Kiszka wrote:

Jan Kiszka wrote:

Hi Philippe,

I'm afraid this one is serious: let the attached migration stress test
run on likely any Xenomai since 2.0, preferably with
CONFIG_XENO_OPT_DEBUG on. Will give a nice crash sooner or later (I'm
trying to set up a serial console now).

Confirmed here. My test box went through some nifty triple salto out of
the window running this frag for 2mn or so. Actually, the semop
handshake is not even needed to cause the crash. At first sight, it
looks like a migration issue taking place during the critical phase when
a shadow thread switches back to Linux to terminate.

As it took some time to persuade my box to not just reboot but to give a
message, I'm posting here the kernel dump of the P-III running

Xenomai: starting native API services.
ce649fb4 ce648000 00000b17 00000202 c0139246 cdf2819c cdf28070 0b12d310
      00000037 ce648000 00000000 c02f0700 00009a28 00000000 b7e94a70
      00000000 ce648000 c0102fcb b7e94a70 bfed63dc b7faf4b0 bfed63c8
Call Trace:
[<c0139246>] __ipipe_dispatch_event+0x96/0x130
[<c0102fcb>] work_resched+0x6/0x1c
Xenomai: fatal: blocked thread migration[22175] rescheduled?!
(status=0x300010, sig=0, prev=watchdog/0[3])

This babe is awaken by Linux while Xeno sees it in a dormant state,
likely after it has terminated. No wonder why things are going wild
after that... Ok, job queued. Thanks.

I think I can explain this warning now: This happens during creation of
a new userspace real-time thread. In the context of the newly created
Linux pthread that is to become a real-time thread, Xenomai first sets
up the real-time part and then calls xnshadow_map. The latter function
does further init and then signals via xnshadow_signal_completion to the
parent Linux thread (the caller of rt_task_create e.g.) that the thread
is up. This happens before xnshadow_harden, i.e. still in preemptible
linux context.

The signalling should normally do not cause a reschedule as the caller -
the to-be-mapped linux pthread - has higher prio than the woken up

Xeno never assumes this.

 And Xenomai implicitly assumes with this fatal-test above that
there is no preemption! But it can happen: the watchdog thread of linux
does preempt here. So, I think it's a false positive.

This is wrong. This check is not related to Linux preemption at all; it makes sure that control over any shadow is shared in a strictly _mutually exclusive_ way, so that a thread blocked at Xenomai level may not not be seen as runnable by Linux either. Disabling it only makes things worse since the scheduling state is obviously corrupted when it triggers, and that's the root bug we are chasing right now. You should not draw any conclusion beyond that. Additionally, keep in mind that Xeno has already run over some PREEMPT_RT patches, for which an infinite number of CPUs is assumed over a fine-grained code base, which induces maximum preemption probabilities.

I disabled this particular warning and came a bit further:

I-pipe: Domain Xenomai registered.
Xenomai: hal/x86 started.
Xenomai: real-time nucleus v2.1 (Surfing With The Alien) loaded.
Xenomai: starting native API services.
Unable to handle kernel paging request at virtual address 75c08732
 printing eip:
*pde = 00000000
Oops: 0000 [#1]
Modules linked in: xeno_native xeno_nucleus eepro100 mii
CPU:    0
EIP:    0060:[<d0acec80>]    Not tainted VLI
EFLAGS: 00010086   (
EIP is at xnpod_schedule+0x790/0xcf0 [xeno_nucleus]
eax: 8005003b   ebx: d09c1a60   ecx: 75c08500   edx: d0ae441c
esi: d0ae4210   edi: ceab1f28   ebp: ceab1f28   esp: ceab1ef4
ds: 007b   es: 007b   ss: 0068
I-pipe domain Xenomai
Stack: 00000096 00000001 c039cce0 0000000e ceab1f28 00000002 ceab1f20
       00000000 cee1ba90 0000000e 00000004 c0103224 00000000 cee00000
       cee1ba90 ce86f700 00000004 cee1b570 0000007b cee1007b ffffffff
Call Trace:
 [<c0103606>] show_stack+0x86/0xc0
 [<c01037a4>] show_registers+0x144/0x200
 [<c01039c7>] die+0xd7/0x1e0
 [<c0286994>] do_page_fault+0x1e4/0x667
 [<c010e094>] __ipipe_handle_exception+0x34/0x80
 [<c0103224>] error_code+0x54/0x70
 [<cee00000>] 0xcee00000
Code: b8 05 e4 01 00 00 39 82 18 02 00 00 74 68 0f 20 c0 83 c8 08 0f 22
c0 8b 4d e8 8b 7d c4 85 ff 8b 49 04 89 4d b8
0f 84 37 fa ff ff <f6> 81 32 02 00 00 40 0f 84 2a fa ff ff b8 00 e0 ff
ff 21 e0 8b
scheduling while atomic: migration/0x00000002/17646
 [<c0103655>] dump_stack+0x15/0x20
 [<c02847fb>] schedule+0x63b/0x720
 [<d0ad6573>] xnshadow_harden+0x83/0x140 [xeno_nucleus]
 [<d0ad6d7a>] xnshadow_wait_barrier+0x7a/0x130 [xeno_nucleus]
 [<d0ad7287>] exec_nucleus_syscall+0x77/0xa0 [xeno_nucleus]
 [<d0ad7769>] losyscall_event+0x139/0x1a0 [xeno_nucleus]
 [<c0139296>] __ipipe_dispatch_event+0x96/0x130
 [<c010dfb7>] __ipipe_syscall_root+0x27/0xc0
 [<c0102e82>] sysenter_past_esp+0x3b/0x67
Xenomai: Switching migration to secondary mode after exception #14 from
user-space at 0xc028450c (pid 17646)
 <3>Debug: sleeping function called from invalid context at
in_atomic():1, irqs_disabled():0
 [<c0103655>] dump_stack+0x15/0x20
 [<c01120b8>] __might_sleep+0x88/0xb0
 [<c01315ad>] futex_wait+0xed/0x2f0
 [<c0131a35>] do_futex+0x45/0x80
 [<c0131ab0>] sys_futex+0x40/0x110
 [<c0102f28>] syscall_call+0x7/0xb

Still problems ahead. I got the impression that the migration path is
not yet well reviewed. :(

Any further ideas welcome!


PS: Tests performed with splhigh/splexit removed from __rt_task_create
(and splnone from gatekeeper_thread) as Philippe privately acknowledged
to be ok. This removes some critical latency source.



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