[PATCH v2][makedumpfile] Fix a data race in multi-threading mode (--num-threads=N)
HAGIO KAZUHITO(萩尾 一仁)
k-hagio-ab at nec.com
Tue Jul 1 21:52:15 PDT 2025
Hi Tao,
On 2025/07/02 13:36, Tao Liu wrote:
> Hi Kazu,
>
> On Wed, Jul 2, 2025 at 12:13 PM HAGIO KAZUHITO(萩尾 一仁)
> <k-hagio-ab at nec.com> wrote:
>>
>> On 2025/07/01 16:59, Tao Liu wrote:
>>> Hi Kazu,
>>>
>>> Thanks for your comments!
>>>
>>> On Tue, Jul 1, 2025 at 7:38 PM HAGIO KAZUHITO(萩尾 一仁) <k-hagio-ab at nec.com> wrote:
>>>>
>>>> Hi Tao,
>>>>
>>>> thank you for the patch.
>>>>
>>>> On 2025/06/25 11:23, Tao Liu wrote:
>>>>> A vmcore corrupt issue has been noticed in powerpc arch [1]. It can be
>>>>> reproduced with upstream makedumpfile.
>>>>>
>>>>> When analyzing the corrupt vmcore using crash, the following error
>>>>> message will output:
>>>>>
>>>>> crash: compressed kdump: uncompress failed: 0
>>>>> crash: read error: kernel virtual address: c0001e2d2fe48000 type:
>>>>> "hardirq thread_union"
>>>>> crash: cannot read hardirq_ctx[930] at c0001e2d2fe48000
>>>>> crash: compressed kdump: uncompress failed: 0
>>>>>
>>>>> If the vmcore is generated without num-threads option, then no such
>>>>> errors are noticed.
>>>>>
>>>>> With --num-threads=N enabled, there will be N sub-threads created. All
>>>>> sub-threads are producers which responsible for mm page processing, e.g.
>>>>> compression. The main thread is the consumer which responsible for
>>>>> writing the compressed data into file. page_flag_buf->ready is used to
>>>>> sync main and sub-threads. When a sub-thread finishes page processing,
>>>>> it will set ready flag to be FLAG_READY. In the meantime, main thread
>>>>> looply check all threads of the ready flags, and break the loop when
>>>>> find FLAG_READY.
>>>>
>>>> I've tried to reproduce the issue, but I couldn't on x86_64.
>>>
>>> Yes, I cannot reproduce it on x86_64 either, but the issue is very
>>> easily reproduced on ppc64 arch, which is where our QE reported.
>>> Recently we have enabled --num-threads=N in rhel by default. N ==
>>> nr_cpus in 2nd kernel, so QE noticed the issue.
>>
>> I see, thank you for the information.
>>
>>>
>>>>
>>>> Do you have any possible scenario that breaks a vmcore? I could not
>>>> think of it only by looking at the code.
>>>
>>> I guess the issue only been observed on ppc might be due to ppc's
>>> memory model, multi-thread scheduling algorithm etc. I'm not an expert
>>> on those. So I cannot give a clear explanation, sorry...
>>
>> ok, I also don't think of how to debug this well..
>>
>>>
>>> The page_flag_buf->ready is an integer that r/w by main and sub
>>> threads simultaneously. And the assignment operation, like
>>> page_flag_buf->ready = 1, might be composed of several assembly
>>> instructions. Without atomic r/w (memory) protection, there might be
>>> racing r/w just within the few instructions, which caused the data
>>> inconsistency. Frankly the ppc assembly consists of more instructions
>>> than x86_64 for the same c code, which enlarged the possibility of
>>> data racing.
>>>
>>> We can observe the issue without the help of crash, just compare the
>>> binary output of vmcore generated from the same core file, and
>>> compress it with or without --num-threads option. Then compare it with
>>> "cmp vmcore1 vmcore2" cmdline, and cmp will output bytes differ for
>>> the 2 vmcores, and this is unexpected.
>>>
>>>>
>>>> and this is just out of curiosity, is the issue reproduced with
>>>> makedumpfile compiled with -O0 too?
>>>
>>> Sorry, I haven't done the -O0 experiment, I can do it tomorrow and
>>> share my findings...
>>
>> Thanks, we have to fix this anyway, I want a clue to think about a
>> possible scenario..
>
> 1) Compiled with -O2 flag:
>
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out1
> Copying data : [100.0 %] /
> eta: 0s
>
> The dumpfile is saved to /tmp/out1.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d
> 31 -l ~/vmcore /tmp/out2
> Copying data : [100.0 %] |
> eta: 0s
> Copying data : [100.0 %] \
> eta: 0s
>
> The dumpfile is saved to /tmp/out2.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# cd /tmp
> [root at ibm-p10-01-lp45 tmp]# cmp out1 out2
> out1 out2 differ: byte 20786414, line 108064
>
> 2) Compiled with -O0 flag:
>
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out3
> Copying data : [100.0 %] /
> eta: 0s
>
> The dumpfile is saved to /tmp/out3.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d
> 31 -l ~/vmcore /tmp/out4
> Copying data : [100.0 %] |
> eta: 0s
> Copying data : [100.0 %] \
> eta: 0s
>
> The dumpfile is saved to /tmp/out4.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# cd /tmp
> [root at ibm-p10-01-lp45 tmp]# cmp out3 out4
> out3 out4 differ: byte 23948282, line 151739
>
> Looks to me the O0/O2 have no difference for this case. If no problem,
> the /tmp/outX generated from both single/multi thread should be
> exactly the same, however the cmp reports there are differences. With
> the v2 patch applied, there is no such difference:
>
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile -d 31 -l ~/vmcore /tmp/out5
> Copying data : [100.0 %] /
> eta: 0s
>
> The dumpfile is saved to /tmp/out5.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# ./makedumpfile --num-threads=2 -d
> 31 -l ~/vmcore /tmp/out6
> Copying data : [100.0 %] |
> eta: 0s
> Copying data : [100.0 %] \
> eta: 0s
>
> The dumpfile is saved to /tmp/out6.
>
> makedumpfile Completed.
> [root at ibm-p10-01-lp45 makedumpfile]# cmp /tmp/out5 /tmp/out6
> [root at ibm-p10-01-lp45 makedumpfile]#
thank you for testing! sorry one more thing,
does --num-threads=1 break the vmcore?
Thanks,
Kazu
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