head -r317820: An example TARGET_ARCH=powerpc panic with interesting information reported (PowerMac G5 so-called "Quad Core" context)

classic Classic list List threaded Threaded
1 message Options
Reply | Threaded
Open this post in threaded view
|

head -r317820: An example TARGET_ARCH=powerpc panic with interesting information reported (PowerMac G5 so-called "Quad Core" context)

Mark Millard-2
Context:

The context is a TARGET_ARCH=powerpc build used on an
old PowerMac G5 so-called "Quad Core". Currently
I'm using head -r317820 for investigating.

I've been having problems with production-style kernel
builds for TARGET_ARCH=powerpc getting occasional
fatal kernel traps --but the debug kernel works and
does not report anything odd. I see this even with pure
gcc 4.2.1 based builds, not just my clang experiments.
Also all the TARGET_ARCH=powerpc64 builds seem to work
fine on the same machines.

All the trap reports indicate pid 11 and one of the cpu
idle threads, more cpu 0 than the others but not limited
to one cpu/thread.


Interesting evidence based on the trap frame:

I caught a data storage interrupt fatal kernel trap from
which the ddb information is interesting . . .

For the fatal kernel trap notice for pid 11's cpu0
thread (there is a "timeout stopping cpus"):

srr0 indicates 0x5e58e0 which is:

sched_choose+0x100: lwz r30, -0x8(r11)

in the build then in use. But lr is listed
as:

lr 0x5358d0, which is before srr0, lr
pointing at:

sched_choose+0xf0: lwz r0,4(r11)

***
Does this ordering of lr vs. srr0 content make
sense? Is it evidence of a problem?
***

The code in the area looks like:

005358b8 <sched_choose+0xd8> li      r0,-1
005358bc <sched_choose+0xdc> stb     r0,152(r29)
005358c0 <sched_choose+0xe0> mfsprg  r9,0
005358c4 <sched_choose+0xe4> lwz     r28,4(r9)
005358c8 <sched_choose+0xe8> mr      r3,r28
005358cc <sched_choose+0xec> lwz     r11,0(r1)
=> 005358d0 <sched_choose+0xf0> lwz     r0,4(r11)
005358d4 <sched_choose+0xf4> mtlr    r0
005358d8 <sched_choose+0xf8> lwz     r28,-16(r11)
005358dc <sched_choose+0xfc> lwz     r29,-12(r11)
=> 005358e0 <sched_choose+0x100> lwz     r30,-8(r11)
005358e4 <sched_choose+0x104> lwz     r31,-4(r11)
005358e8 <sched_choose+0x108> mr      r1,r11
005358ec <sched_choose+0x10c> blr

The r3, r28, r11, r9, and r0 values in the
trap frame do not seem to match the code.

For example at 005358d0 the above
would have r3=r28 but the "show reg"
shows:

r3         0x0
r28 0x7c2803c6

The r1 value and vmcore.3 content indicate
0x0147d6c0 for the later "lwz r28,-16(r11)".

"show reg" reported both of the following for
the trap frame:

r1  0xdf5e58c0
r11 0xdf5e58c0

which would indicate r11 not being from the
result of "lwz r11,0(r1)". vmcore.3 content
indicates two things:

A) The trap frame shows the 0xdf5e58c0 for r11.
B) 0(r1) would have been 0xdf5e58e0 .

In vmcore.3 0xdf5e58e0 has an associated lr
value next to it: 0x5358ac, matching up with
the return from the last bl in sched_choose:

005358a0 <sched_choose+0xc0> b       005358ac <sched_choose+0xcc>
005358a4 <sched_choose+0xc4> mr      r4,r28
005358a8 <sched_choose+0xc8> bl      00500010 <runq_remove>
005358ac <sched_choose+0xcc> lbz     r0,622(r28)

So it looks as expected in vmcore.3 . Then either:

a) The trap from is from prior to the
   "lwz r11,0(r1)" result.

b) The 0(r1) access was not based on
   coherent/up-to-date cache/memory
   contents and so filled r11 with junk.

c) The r11 value in the trap frame has been
   corrupted some other way.

The trap frame shows:

r9 0x1f8

But 0x1f8 is much smaller than what I
see on a live system for sprg0: more
like 0xf65f00 . And the 4(r9) would
not get to a 0x7c2803c6 value from
what I see in vmcore.3 .

The trap frame's

r0 0x4bfca6c1

does not fit with either "li r0,-1" nor
the code:

=> 005358d0 <sched_choose+0xf0> lwz     r0,4(r11)
005358d4 <sched_choose+0xf4> mtlr    r0

(The trap frame reports lr as 0x5358d0.)

(r0's value only exists in two places in
vmcore.3, one being the trap frame.)

The vmcore.3 and r1 would suggest the result
of:

005358cc <sched_choose+0xec> lwz     r11,0(r1)
=> 005358d0 <sched_choose+0xf0> lwz     r0,4(r11)
005358d4 <sched_choose+0xf4> mtlr    r0
005358d8 <sched_choose+0xf8> lwz     r28,-16(r11)
005358dc <sched_choose+0xfc> lwz     r29,-12(r11)
=> 005358e0 <sched_choose+0x100> lwz     r30,-8(r11)
005358e4 <sched_choose+0x104> lwz     r31,-4(r11)
005358e8 <sched_choose+0x108> mr      r1,r11

should be the sequence of register assignments:

r11 = 0xdf5e58e0
r0  = 0x00500460
lr  = 0x00500460
r28 = 0x0147d6c0
r29 = 0x00d4d7c8
r30 = 0x00d1d6c4
r31 = 0xdf5e58e0
r1  = 0xdf5e58e0

None of which show up in the trap frame.
But even the trap frame's 005358d0 would
suggest the first of the above.

As for the virtual address reported
for the failure:

virtual address = 0x7c2803ba

There is only one register listed with a
value near that:

r28 0x7c2803a6
(This value is in vmcore.3 something like
677 times.)

and: 0x7c2803ba - 0x7c2803a6 == 0x14 (20 decimal).

But it does not fit for the code getting to
0x7c2803ba from r28. 0x7c2803ba only exists
in 3 or so places in vmcore.3 (including the
trap frame instance).

I'm not sure how 0x7c2803ba ended up in the
dar (or its spot in the trap frame) in
vmcore.3 .


Supporting detail:

Looking around in the vmcore.3 file showed that
the area I found with the proper back chain and
lr links was present (this was debug.minidump=0),
complete with having one of 3 instances of the
failing virtual memory address (0x7c2803ba) near
by --happening to be the dar value from the trap
frame when I looked in detail.

(Of course in vmcore.3 I'm seeing physical
memory addresses offset by the dump's header
size for locations of memory. So I'm learning
what the mapping was by finding the region
with the content.)


Notes:


I have evidence that changes in the
memory layout of the kernel (such as
by adding something to potentially use
in investigating) changes the details
of the failure behavior. I have had
contexts where the failures would happen
when the PowerMac involved was booted
but not in active use. In other contexts
I've had that not fail but something like
"find / -name dog -print | more" leads
to failure before completing. In the
other context such things completed just
fine.


libkvm was never updated to deal with
the powerpc and powerpc64 relocatable
kernel format changes. On the PowerMac
it seems to leave the kernel at its
default place but the format is ET_DYN.

More than just testing for ET_DYN is
required in libkvm. And it appears that
for powerpc and powerpc64 it never
supported debug.minidump=0 . The open
Bugzilla 219153 reports this issue
and has more notes.

===
Mark Millard
markmi at dsl-only.net

_______________________________________________
[hidden email] mailing list
https://lists.freebsd.org/mailman/listinfo/freebsd-ppc
To unsubscribe, send any mail to "[hidden email]"