Re: [PATCH v5 0/4] arm64: more improvement of bt -f

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----- Original Message -----
> 
>  
> Hi Takahiro,
> 
> I applied patches 1/2 and 2/2 from the v5 patchset.  But I can't
> believe the results are what you intended?

Obviously I meant 1/4 and 2/4 above.

However, I was under the impression that the 3/4 patch was a standalone
patch that only served to change the text address displayed, and that
"adding this patch was a discussion topic":  ​

>  On arm64, the link register (LR) holds a return address, which is the one
>  just after a branch instruction. So using a saved lr as PC for backtracing
>  might cause some confusion.
>  For example, in kernel/entry.S,
>  work_resched:
>      ...
>      bl schedule
>
>  ret_to_user:
>      ...
>
>  The current code shows "ret_o_user", instead of "work_resched",
>  as a caller of schedule().
>
>  This patch corrects a PC by decrementing it by 4.
>  But please note that this change may also make people a bit confused
>  because a value of LR in the stack dump of "bt -f" doesn't match with
>  an address in one-line summary.
>
>   #2 [ffffcc7511407eb0] schedule at ffff0000d628aee0
>      ffffcc7511407eb0: ffffcc6d22f23080 ffff0000d5b44d6c  <= LR
>      ffffcc7511407ec0: ffffcc7511407ed0 0000000000000000
>   #3 [ffffcc7511407ed0] work_resched at ffff0000d5b44d68  <= correcrted PC
>
>  Signed-off-by: AKASHI Takahiro <takahiro akashi linaro org>

...and as you subsequently mentioned, "adding this patch was a discussion topic".

But anyway, for the hell of it, I subsequently applied 3/4, and now I at least see
the IPI exception frames:


crash> bt -a
PID: 0      TASK: ffff000008dcd900  CPU: 0   COMMAND: "swapper/0"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff000008dc3f10  PSTATE: 60400149
    X29: ffff000008dc3f10  X28: ffff000008dc0000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff000008dc0000  X22: ffff000008bd0270  X21: ffff000008dc0000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: 00000000a632f641
    X17: 0000ffff7da57880  X16: ffff0000081d9838  X15: 00000000383a0a79
    X14: 00000000b2b0b162  X13: 000000005f2cbeec  X12: 0000000000045a9e
    X11: ffff8000213bd800  X10: 0000000000000850   X9: ffff000008dc0000
     X8: 000000010000aa07   X7: 000000000000003d   X6: 0015752a00000000
     X5: 0100000000000000   X4: 00000000000001c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff000008dc3f10] arch_cpu_idle at ffff0000080857bc
 #1 [ffff000008dc3f20] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff000008dc3f80] rest_init at ffff0000087c792c
 #3 [ffff000008dc3fa0] start_kernel at ffff000008b10b6c

PID: 0      TASK: ffff8000218c0c80  CPU: 1   COMMAND: "swapper/1"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff8000218cff60  PSTATE: 60000349
    X29: ffff8000218cff60  X28: ffff8000218cc000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff8000218cc000  X22: ffff000008bd0270  X21: ffff8000218cc000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
    X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000005
    X14: 0000000c80096000  X13: ffff8000212b6600  X12: 0000000000047ae2
    X11: ffff800021089980  X10: 0000000000000850   X9: ffff8000218cc000
     X8: 000000010000aa07   X7: 0000000000000243   X6: 0015752a00000000
     X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff8000218cff60] arch_cpu_idle at ffff0000080857bc
 #1 [ffff8000218cff70] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff8000218cffd0] secondary_start_kernel at ffff00000808e1e8

PID: 1324   TASK: ffff80002018be80  CPU: 2   COMMAND: "dhry"
     PC: 00000000004016a4   LR: 00000000004016a4   SP: 0000ffffc10c40a0
    X29: 0000ffffc10c40a0  X28: 0000000000000000  X27: 0000000000000000
    X26: 0000000000000000  X25: 0000000000402138  X24: 00000000004021f0
    X23: 0000000000000000  X22: 0000000000000000  X21: 00000000004001a0
    X20: 0000000000000000  X19: 0000000000000000  X18: 0000000000000000
    X17: 0000000000000001  X16: 0000000000000000  X15: 0000000000493000
    X14: 0000000000498000  X13: ffffffffffffffff  X12: 0000000000000005
    X11: 000000000000001e  X10: 0101010101010101   X9: fffffffff59a9190
     X8: 7f7f7f7f7f7f7f7f   X7: 1f535226301f2b4c   X6: 00000003001d1000
     X5: 00101d0003000000   X4: 0000000000000000   X3: 4952545320454d4f
     X2: 0000000010c35b40   X1: 0000000000000011   X0: 0000000010c35b40
    ORIG_X0: 0000000000498700  SYSCALLNO: ffffffffffffffff  PSTATE: 20000000

 #0 [user space]

PID: 1223   TASK: ffff800020ef5780  CPU: 3   COMMAND: "sh"
 #0 [ffff800020b0ba70] crash_kexec at ffff00000812b0a8
 #1 [ffff800020b0ba90] die at ffff000008088ce4
 #2 [ffff800020b0bad0] __do_kernel_fault at ffff000008098fa8
 #3 [ffff800020b0bb00] do_page_fault at ffff000008096810
 #4 [ffff800020b0bb60] do_translation_fault at ffff000008096908
 #5 [ffff800020b0bb70] do_mem_abort at ffff000008081288
 #6 [ffff800020b0bc20] el1_da at ffff000008084564
--- <Exception in kernel> ---
     PC: ffff000008457fc8  [sysrq_handle_crash+32]
     LR: ffff000008457fb4  [sysrq_handle_crash+12]
     SP: ffff800020b0bd40  PSTATE: 60400149
    X29: ffff800020b0bd40  X28: ffff800020b08000  X27: ffff0000087e2000
    X26: 0000000000000040  X25: 0000000000000120  X24: 0000000000000015
    X23: 0000000000000000  X22: 0000000000000009  X21: ffff000008e071b0
    X20: 0000000000000063  X19: ffff000008dda000  X18: 0000000000000006
    X17: 0000ffff83d266c0  X16: ffff0000081c68b8  X15: ffff000008e6cc95
    X14: 0000ffff83cc28f0  X13: ffff000008e6c758  X12: ffff000008dda7a0
    X11: 0000000000000106  X10: 0000000000000002   X9: 0000000000000001
     X8: 0000000000000106   X7: 0000000000000000   X6: ffff800022f337a4
     X5: 00000000000047d4   X4: 0000000000000000   X3: 0000000000000000
     X2: ffff000008dda7b8   X1: 0000000000000000   X0: 0000000000000001
    
 #7 [ffff800020b0bd40] sysrq_handle_crash at ffff000008457fc4
 #8 [ffff800020b0bd50] __handle_sysrq at ffff000008458640
 #9 [ffff800020b0bd90] write_sysrq_trigger at ffff000008458abc
#10 [ffff800020b0bdb0] proc_reg_write at ffff000008225418
#11 [ffff800020b0bdd0] __vfs_write at ffff0000081c4770
#12 [ffff800020b0be50] vfs_write at ffff0000081c5584
#13 [ffff800020b0be90] sys_write at ffff0000081c68f8
#14 [ffff800020b0bed0] el0_svc_naked at ffff000008084e6c
--- <Exception in user> ---
     PC: 0000ffff83d7b1e8   LR: 0000ffff83d2d700   SP: 0000fffffe197460
    X29: 0000fffffe197460  X28: 0000000000000000  X27: 0000000000000000
    X26: 000000001e9e40c8  X25: 00000000004c9be8  X24: 00000000004ec000
    X23: 0000000000000001  X22: 0000000000000002  X21: 0000ffff83e02500
    X20: 000000001e9da808  X19: 0000000000000002  X18: 0000000000000015
    X17: 0000ffff83d266c0  X16: 0000000000000000  X15: 0015a60d0a000000
    X14: 0000ffff83cc28f0  X13: 0000000000000018  X12: 0000000000000008
    X11: 0101010101010101  X10: 7f7f7f7f7f7f7f7f   X9: fefefefeff00ff62
     X8: 0000000000000040   X7: fefefefefefefefe   X6: 000000001e9da808
     X5: 0000ffff83d055c8   X4: 0000000000000000   X3: 0000000000000000
     X2: 0000000000000002   X1: 000000001e9da808   X0: 0000000000000001
    ORIG_X0: 0000000000000001  SYSCALLNO: 40  PSTATE: 20000000

PID: 0      TASK: ffff8000218c3200  CPU: 4   COMMAND: "swapper/4"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff8000218dbf60  PSTATE: 60000349
    X29: ffff8000218dbf60  X28: ffff8000218d8000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff8000218d8000  X22: ffff000008bd0270  X21: ffff8000218d8000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
    X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000000
    X14: 0000000000000000  X13: 00000000000003e7  X12: 0000000000000016
    X11: ffff0000087ef470  X10: 0000000000000850   X9: ffff8000218d8000
     X8: ffff8000218c3ab0   X7: 7fffffffffffffff   X6: 0015752a00000000
     X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff8000218dbf60] arch_cpu_idle at ffff0000080857bc
 #1 [ffff8000218dbf70] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff8000218dbfd0] secondary_start_kernel at ffff00000808e1e8

PID: 0      TASK: ffff8000218c3e80  CPU: 5   COMMAND: "swapper/5"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff8000218dff60  PSTATE: 60000349
    X29: ffff8000218dff60  X28: ffff8000218dc000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff8000218dc000  X22: ffff000008bd0270  X21: ffff8000218dc000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
    X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000007
    X14: 0000000000000000  X13: 000000000000039d  X12: 0000000000000000
    X11: ffff0000087ef470  X10: 0000000000000850   X9: ffff8000218dc000
     X8: 000000010000a479   X7: 7fffffffffffffff   X6: 0016694e00000000
     X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff8000218dff60] arch_cpu_idle at ffff0000080857bc
 #1 [ffff8000218dff70] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff8000218dffd0] secondary_start_kernel at ffff00000808e1e8

PID: 0      TASK: ffff8000218c4b00  CPU: 6   COMMAND: "swapper/6"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff8000218ebf60  PSTATE: 60000349
    X29: ffff8000218ebf60  X28: ffff8000218e8000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff8000218e8000  X22: ffff000008bd0270  X21: ffff8000218e8000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
    X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 000000000000000a
    X14: 0000000000000000  X13: 00000000000003a1  X12: 0000000000000000
    X11: ffff8000213a0d00  X10: 0000000000000850   X9: ffff8000218e8000
     X8: ffff8000218c53b0   X7: 00000000000001c4   X6: 002887fa00000000
     X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff8000218ebf60] arch_cpu_idle at ffff0000080857bc
 #1 [ffff8000218ebf70] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff8000218ebfd0] secondary_start_kernel at ffff00000808e1e8

PID: 0      TASK: ffff8000218c5780  CPU: 7   COMMAND: "swapper/7"
     PC: ffff0000080857c0  [arch_cpu_idle+16]
     LR: ffff0000080857bc  [arch_cpu_idle+12]
     SP: ffff8000218eff60  PSTATE: 60400149
    X29: ffff8000218eff60  X28: ffff8000218ec000  X27: 0000000000000000
    X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
    X23: ffff8000218ec000  X22: ffff000008bd0270  X21: ffff8000218ec000
    X20: ffff000008dc5b88  X19: 0000000000000000  X18: fffffffffffffffb
    X17: 0000ffff9dc8ba48  X16: ffff0000081da048  X15: 003b9aca00000000
    X14: 0000000000000000  X13: 0000000000000001  X12: 0000000000000000
    X11: ffff800021089980  X10: 0000000000000850   X9: ffff8000218ec000
     X8: 000000010000a280   X7: ffff8000209b39c8   X6: ffff8000209b39c9
     X5: 0100000000000000   X4: 00000000000001c0   X3: 0000000000000000
     X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
    
 #0 [ffff8000218eff60] arch_cpu_idle at ffff0000080857bc
 #1 [ffff8000218eff70] cpu_startup_entry at ffff0000080f26c8
 #2 [ffff8000218effd0] secondary_start_kernel at ffff00000808e1e8
crash>

But why do you think that it is an improvement to leave out the transition
to the IRQ stack?  

In any case, I haven't even started looking at the "bt -f" part of the patch
because quite frankly, this patchset is so complex that I haven't even
begun to try to understand it.  When you said it would be "easier to 
maintain", well, perhaps for *you* maybe, but certainly not for me!  Without
the kernel's backtrace code from which the current code is based, I don't have
anything to work from anymore.  I'm really not sure whether the coverage of the 
"corner cases" you referred to make this effort worth it.  I would like to see examples
of how the current code fails.  But anyway, I will continue to test it to see 
if there actually is any significant upgrade from what we already have in place.
>From a kernel debugging perspective, all we really need is a basic unwinder, 
and optionally the full dump of the stack data in between those frames.  And the
current code does do that at a minimum, and there's much to be said for simplicity.

Dave
  


  





> 
> For example, taking the 4.6 vmcore that you gave to me, here is the
> current crash utility's output of "bt -a", where the crashing task
> entered crash_kexec() via the sysrq-c page fault exception, and the
> tasks on the other cpus have all entered crash_save_cpu() on their
> IRQ stack as a result of the shutdown IPI, one from user-space and
> the others from the kernel:
>   
>   crash> bt -a
>   PID: 0      TASK: ffff000008dcd900  CPU: 0   COMMAND: "swapper/0"
>    #0 [ffff800022f42e50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022f43010] handle_IPI at ffff00000808e718
>    #2 [ffff800022f43040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022f43080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff000008dc3f10  PSTATE: 60400149
>       X29: ffff000008dc3f10  X28: ffff000008dc0000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff000008dc0000  X22: ffff000008bd0270  X21: ffff000008dc0000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: 00000000a632f641
>       X17: 0000ffff7da57880  X16: ffff0000081d9838  X15: 00000000383a0a79
>       X14: 00000000b2b0b162  X13: 000000005f2cbeec  X12: 0000000000045a9e
>       X11: ffff8000213bd800  X10: 0000000000000850   X9: ffff000008dc0000
>        X8: 000000010000aa07   X7: 000000000000003d   X6: 0015752a00000000
>        X5: 0100000000000000   X4: 00000000000001c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff000008dc3f10] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff000008dc3f20] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff000008dc3f80] rest_init at ffff0000087c7930
>    #7 [ffff000008dc3fa0] start_kernel at ffff000008b10b70
>   
>   PID: 0      TASK: ffff8000218c0c80  CPU: 1   COMMAND: "swapper/1"
>    #0 [ffff800022f56e50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022f57010] handle_IPI at ffff00000808e718
>    #2 [ffff800022f57040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022f57080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff8000218cff60  PSTATE: 60000349
>       X29: ffff8000218cff60  X28: ffff8000218cc000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff8000218cc000  X22: ffff000008bd0270  X21: ffff8000218cc000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
>       X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000005
>       X14: 0000000c80096000  X13: ffff8000212b6600  X12: 0000000000047ae2
>       X11: ffff800021089980  X10: 0000000000000850   X9: ffff8000218cc000
>        X8: 000000010000aa07   X7: 0000000000000243   X6: 0015752a00000000
>        X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff8000218cff60] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff8000218cff70] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff8000218cffd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 1324   TASK: ffff80002018be80  CPU: 2   COMMAND: "dhry"
>    #0 [ffff800022f6ae50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022f6b010] handle_IPI at ffff00000808e718
>    #2 [ffff800022f6b040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022f6b080] el0_irq_naked at ffff000008084c4c
>   --- <IRQ stack> ---
>        PC: 00000000004016a4   LR: 00000000004016a4   SP: 0000ffffc10c40a0
>       X29: 0000ffffc10c40a0  X28: 0000000000000000  X27: 0000000000000000
>       X26: 0000000000000000  X25: 0000000000402138  X24: 00000000004021f0
>       X23: 0000000000000000  X22: 0000000000000000  X21: 00000000004001a0
>       X20: 0000000000000000  X19: 0000000000000000  X18: 0000000000000000
>       X17: 0000000000000001  X16: 0000000000000000  X15: 0000000000493000
>       X14: 0000000000498000  X13: ffffffffffffffff  X12: 0000000000000005
>       X11: 000000000000001e  X10: 0101010101010101   X9: fffffffff59a9190
>        X8: 7f7f7f7f7f7f7f7f   X7: 1f535226301f2b4c   X6: 00000003001d1000
>        X5: 00101d0003000000   X4: 0000000000000000   X3: 4952545320454d4f
>        X2: 0000000010c35b40   X1: 0000000000000011   X0: 0000000010c35b40
>       ORIG_X0: 0000000000498700  SYSCALLNO: ffffffffffffffff  PSTATE:
>       20000000
>   
>   PID: 1223   TASK: ffff800020ef5780  CPU: 3   COMMAND: "sh"
>    #0 [ffff800020b0ba70] crash_kexec at ffff00000812b0ac
>    #1 [ffff800020b0ba90] die at ffff000008088ce8
>    #2 [ffff800020b0bad0] __do_kernel_fault at ffff000008098fac
>    #3 [ffff800020b0bb00] do_page_fault at ffff000008096814
>    #4 [ffff800020b0bb60] do_translation_fault at ffff00000809690c
>    #5 [ffff800020b0bb70] do_mem_abort at ffff00000808128c
>    #6 [ffff800020b0bd40] el1_da at ffff000008084568
>        PC: ffff000008457fc8  [sysrq_handle_crash+32]
>        LR: ffff000008457fb4  [sysrq_handle_crash+12]
>        SP: ffff800020b0bd40  PSTATE: 60400149
>       X29: ffff800020b0bd40  X28: ffff800020b08000  X27: ffff0000087e2000
>       X26: 0000000000000040  X25: 0000000000000120  X24: 0000000000000015
>       X23: 0000000000000000  X22: 0000000000000009  X21: ffff000008e071b0
>       X20: 0000000000000063  X19: ffff000008dda000  X18: 0000000000000006
>       X17: 0000ffff83d266c0  X16: ffff0000081c68b8  X15: ffff000008e6cc95
>       X14: 0000ffff83cc28f0  X13: ffff000008e6c758  X12: ffff000008dda7a0
>       X11: 0000000000000106  X10: 0000000000000002   X9: 0000000000000001
>        X8: 0000000000000106   X7: 0000000000000000   X6: ffff800022f337a4
>        X5: 00000000000047d4   X4: 0000000000000000   X3: 0000000000000000
>        X2: ffff000008dda7b8   X1: 0000000000000000   X0: 0000000000000001
>       ORIG_X0: ffff000008dda000  SYSCALLNO: ffff80002104d418
>    #7 [ffff800020b0bd50] __handle_sysrq at ffff000008458644
>    #8 [ffff800020b0bd90] write_sysrq_trigger at ffff000008458ac0
>    #9 [ffff800020b0bdb0] proc_reg_write at ffff00000822541c
>   #10 [ffff800020b0bdd0] __vfs_write at ffff0000081c4774
>   #11 [ffff800020b0be50] vfs_write at ffff0000081c5588
>   #12 [ffff800020b0be90] sys_write at ffff0000081c68fc
>   #13 [ffff800020b0bed0] el0_svc_naked at ffff000008084e70
>        PC: 0000ffff83d7b1e8   LR: 0000ffff83d2d700   SP: 0000fffffe197460
>       X29: 0000fffffe197460  X28: 0000000000000000  X27: 0000000000000000
>       X26: 000000001e9e40c8  X25: 00000000004c9be8  X24: 00000000004ec000
>       X23: 0000000000000001  X22: 0000000000000002  X21: 0000ffff83e02500
>       X20: 000000001e9da808  X19: 0000000000000002  X18: 0000000000000015
>       X17: 0000ffff83d266c0  X16: 0000000000000000  X15: 0015a60d0a000000
>       X14: 0000ffff83cc28f0  X13: 0000000000000018  X12: 0000000000000008
>       X11: 0101010101010101  X10: 7f7f7f7f7f7f7f7f   X9: fefefefeff00ff62
>        X8: 0000000000000040   X7: fefefefefefefefe   X6: 000000001e9da808
>        X5: 0000ffff83d055c8   X4: 0000000000000000   X3: 0000000000000000
>        X2: 0000000000000002   X1: 000000001e9da808   X0: 0000000000000001
>       ORIG_X0: 0000000000000001  SYSCALLNO: 40  PSTATE: 20000000
>   
>   PID: 0      TASK: ffff8000218c3200  CPU: 4   COMMAND: "swapper/4"
>    #0 [ffff800022f92e50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022f93010] handle_IPI at ffff00000808e718
>    #2 [ffff800022f93040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022f93080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff8000218dbf60  PSTATE: 60000349
>       X29: ffff8000218dbf60  X28: ffff8000218d8000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff8000218d8000  X22: ffff000008bd0270  X21: ffff8000218d8000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
>       X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000000
>       X14: 0000000000000000  X13: 00000000000003e7  X12: 0000000000000016
>       X11: ffff0000087ef470  X10: 0000000000000850   X9: ffff8000218d8000
>        X8: ffff8000218c3ab0   X7: 7fffffffffffffff   X6: 0015752a00000000
>        X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff8000218dbf60] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff8000218dbf70] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff8000218dbfd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c3e80  CPU: 5   COMMAND: "swapper/5"
>    #0 [ffff800022fa6e50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022fa7010] handle_IPI at ffff00000808e718
>    #2 [ffff800022fa7040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022fa7080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff8000218dff60  PSTATE: 60000349
>       X29: ffff8000218dff60  X28: ffff8000218dc000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff8000218dc000  X22: ffff000008bd0270  X21: ffff8000218dc000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
>       X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 0000000000000007
>       X14: 0000000000000000  X13: 000000000000039d  X12: 0000000000000000
>       X11: ffff0000087ef470  X10: 0000000000000850   X9: ffff8000218dc000
>        X8: 000000010000a479   X7: 7fffffffffffffff   X6: 0016694e00000000
>        X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff8000218dff60] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff8000218dff70] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff8000218dffd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c4b00  CPU: 6   COMMAND: "swapper/6"
>    #0 [ffff800022fbae50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022fbb010] handle_IPI at ffff00000808e718
>    #2 [ffff800022fbb040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022fbb080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff8000218ebf60  PSTATE: 60000349
>       X29: ffff8000218ebf60  X28: ffff8000218e8000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff8000218e8000  X22: ffff000008bd0270  X21: ffff8000218e8000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: 000000000000016d
>       X17: 0000007f8b122780  X16: ffffffc0001adf68  X15: 000000000000000a
>       X14: 0000000000000000  X13: 00000000000003a1  X12: 0000000000000000
>       X11: ffff8000213a0d00  X10: 0000000000000850   X9: ffff8000218e8000
>        X8: ffff8000218c53b0   X7: 00000000000001c4   X6: 002887fa00000000
>        X5: 0100000000000000   X4: 00000000000003c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff8000218ebf60] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff8000218ebf70] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff8000218ebfd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c5780  CPU: 7   COMMAND: "swapper/7"
>    #0 [ffff800022fcee50] crash_save_cpu at ffff00000812ae44
>    #1 [ffff800022fcf010] handle_IPI at ffff00000808e718
>    #2 [ffff800022fcf040] gic_handle_irq at ffff0000080815f8
>    #3 [ffff800022fcf080] el1_irq at ffff000008084720
>   --- <IRQ stack> ---
>        PC: ffff0000080857c0  [arch_cpu_idle+16]
>        LR: ffff0000080857bc  [arch_cpu_idle+12]
>        SP: ffff8000218eff60  PSTATE: 60400149
>       X29: ffff8000218eff60  X28: ffff8000218ec000  X27: 0000000000000000
>       X26: ffff000008dc5000  X25: ffff000008dc5c1c  X24: ffff000008dc5000
>       X23: ffff8000218ec000  X22: ffff000008bd0270  X21: ffff8000218ec000
>       X20: ffff000008dc5b88  X19: 0000000000000000  X18: fffffffffffffffb
>       X17: 0000ffff9dc8ba48  X16: ffff0000081da048  X15: 003b9aca00000000
>       X14: 0000000000000000  X13: 0000000000000001  X12: 0000000000000000
>       X11: ffff800021089980  X10: 0000000000000850   X9: ffff8000218ec000
>        X8: 000000010000a280   X7: ffff8000209b39c8   X6: ffff8000209b39c9
>        X5: 0100000000000000   X4: 00000000000001c0   X3: 0000000000000000
>        X2: 0000000000000001   X1: 0000000000000000   X0: 0000000000000000
>       ORIG_X0: 0000000000000000  SYSCALLNO: 7fffffffffffffff
>    #4 [ffff8000218eff60] arch_cpu_idle at ffff0000080857c0
>    #5 [ffff8000218eff70] cpu_startup_entry at ffff0000080f26cc
>    #6 [ffff8000218effd0] secondary_start_kernel at ffff00000808e1ec
>   crash>
> 
> And that's precisely what we want to see.  (other than perhaps the
> in-kernel exception frame's dump of the ORIG_X0 and SYSCALL registers)
> 
> Now, with v5's patches 1/2 and 2/2 patches applied, this is what gets
> displayed:
> 
>   crash> bt -a
>   PID: 0      TASK: ffff000008dcd900  CPU: 0   COMMAND: "swapper/0"
>    #0 [ffff000008dc3f10] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff000008dc3f20] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff000008dc3f80] rest_init at ffff0000087c7930
>    #3 [ffff000008dc3fa0] start_kernel at ffff000008b10b70
>   
>   PID: 0      TASK: ffff8000218c0c80  CPU: 1   COMMAND: "swapper/1"
>    #0 [ffff8000218cff60] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff8000218cff70] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff8000218cffd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 1324   TASK: ffff80002018be80  CPU: 2   COMMAND: "dhry"
>        PC: 00000000004016a4   LR: 00000000004016a4   SP: 0000ffffc10c40a0
>       X29: 0000ffffc10c40a0  X28: 0000000000000000  X27: 0000000000000000
>       X26: 0000000000000000  X25: 0000000000402138  X24: 00000000004021f0
>       X23: 0000000000000000  X22: 0000000000000000  X21: 00000000004001a0
>       X20: 0000000000000000  X19: 0000000000000000  X18: 0000000000000000
>       X17: 0000000000000001  X16: 0000000000000000  X15: 0000000000493000
>       X14: 0000000000498000  X13: ffffffffffffffff  X12: 0000000000000005
>       X11: 000000000000001e  X10: 0101010101010101   X9: fffffffff59a9190
>        X8: 7f7f7f7f7f7f7f7f   X7: 1f535226301f2b4c   X6: 00000003001d1000
>        X5: 00101d0003000000   X4: 0000000000000000   X3: 4952545320454d4f
>        X2: 0000000010c35b40   X1: 0000000000000011   X0: 0000000010c35b40
>       ORIG_X0: 0000000000498700  SYSCALLNO: ffffffffffffffff  PSTATE:
>       20000000
>   
>    #0 [user space]
>   
>   PID: 1223   TASK: ffff800020ef5780  CPU: 3   COMMAND: "sh"
>    #0 [ffff800020b0ba70] crash_kexec at ffff00000812b0ac
>    #1 [ffff800020b0ba90] die at ffff000008088ce8
>    #2 [ffff800020b0bad0] __do_kernel_fault at ffff000008098fac
>    #3 [ffff800020b0bb00] do_page_fault at ffff000008096814
>    #4 [ffff800020b0bb60] do_translation_fault at ffff00000809690c
>    #5 [ffff800020b0bb70] do_mem_abort at ffff00000808128c
>    #6 [ffff800020b0bc20] el1_da at ffff000008084568
>   --- <Exception in kernel> ---
>        PC: ffff000008457fc8  [sysrq_handle_crash+32]
>        LR: ffff000008457fb4  [sysrq_handle_crash+12]
>        SP: ffff800020b0bd40  PSTATE: 60400149
>       X29: ffff800020b0bd40  X28: ffff800020b08000  X27: ffff0000087e2000
>       X26: 0000000000000040  X25: 0000000000000120  X24: 0000000000000015
>       X23: 0000000000000000  X22: 0000000000000009  X21: ffff000008e071b0
>       X20: 0000000000000063  X19: ffff000008dda000  X18: 0000000000000006
>       X17: 0000ffff83d266c0  X16: ffff0000081c68b8  X15: ffff000008e6cc95
>       X14: 0000ffff83cc28f0  X13: ffff000008e6c758  X12: ffff000008dda7a0
>       X11: 0000000000000106  X10: 0000000000000002   X9: 0000000000000001
>        X8: 0000000000000106   X7: 0000000000000000   X6: ffff800022f337a4
>        X5: 00000000000047d4   X4: 0000000000000000   X3: 0000000000000000
>        X2: ffff000008dda7b8   X1: 0000000000000000   X0: 0000000000000001
>       
>    #7 [ffff800020b0bd40] sysrq_handle_crash at ffff000008457fc8
>    #8 [ffff800020b0bd50] __handle_sysrq at ffff000008458644
>    #9 [ffff800020b0bd90] write_sysrq_trigger at ffff000008458ac0
>   #10 [ffff800020b0bdb0] proc_reg_write at ffff00000822541c
>   #11 [ffff800020b0bdd0] __vfs_write at ffff0000081c4774
>   #12 [ffff800020b0be50] vfs_write at ffff0000081c5588
>   #13 [ffff800020b0be90] sys_write at ffff0000081c68fc
>   #14 [ffff800020b0bed0] el0_svc_naked at ffff000008084e70
>   --- <Exception in user> ---
>        PC: 0000ffff83d7b1e8   LR: 0000ffff83d2d700   SP: 0000fffffe197460
>       X29: 0000fffffe197460  X28: 0000000000000000  X27: 0000000000000000
>       X26: 000000001e9e40c8  X25: 00000000004c9be8  X24: 00000000004ec000
>       X23: 0000000000000001  X22: 0000000000000002  X21: 0000ffff83e02500
>       X20: 000000001e9da808  X19: 0000000000000002  X18: 0000000000000015
>       X17: 0000ffff83d266c0  X16: 0000000000000000  X15: 0015a60d0a000000
>       X14: 0000ffff83cc28f0  X13: 0000000000000018  X12: 0000000000000008
>       X11: 0101010101010101  X10: 7f7f7f7f7f7f7f7f   X9: fefefefeff00ff62
>        X8: 0000000000000040   X7: fefefefefefefefe   X6: 000000001e9da808
>        X5: 0000ffff83d055c8   X4: 0000000000000000   X3: 0000000000000000
>        X2: 0000000000000002   X1: 000000001e9da808   X0: 0000000000000001
>       ORIG_X0: 0000000000000001  SYSCALLNO: 40  PSTATE: 20000000
>   
>   PID: 0      TASK: ffff8000218c3200  CPU: 4   COMMAND: "swapper/4"
>    #0 [ffff8000218dbf60] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff8000218dbf70] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff8000218dbfd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c3e80  CPU: 5   COMMAND: "swapper/5"
>    #0 [ffff8000218dff60] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff8000218dff70] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff8000218dffd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c4b00  CPU: 6   COMMAND: "swapper/6"
>    #0 [ffff8000218ebf60] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff8000218ebf70] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff8000218ebfd0] secondary_start_kernel at ffff00000808e1ec
>   
>   PID: 0      TASK: ffff8000218c5780  CPU: 7   COMMAND: "swapper/7"
>    #0 [ffff8000218eff60] arch_cpu_idle at ffff0000080857c0
>    #1 [ffff8000218eff70] cpu_startup_entry at ffff0000080f26cc
>    #2 [ffff8000218effd0] secondary_start_kernel at ffff00000808e1ec
>   crash>
> 
> What happened?  This can't possibly be what you intended, is it?
> 
> Dave
> 
> --
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>

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