[PATCH v2][makedumpfile] Fix a data race in multi-threading mode (--num-threads=N)

[Tao Liu]

On Fri, Jul 4, 2025 at 6:49 PM HAGIO KAZUHITO(萩尾　一仁) <k-hagio-ab at nec.com> wrote:
>
> On 2025/07/04 7:35, Tao Liu wrote:
> > Hi Petr,
> >
> > On Fri, Jul 4, 2025 at 2:31 AM Petr Tesarik <ptesarik at suse.com> wrote:
> >>
> >> On Tue, 1 Jul 2025 19:59:53 +1200
> >> Tao Liu <ltao at redhat.com> 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.
> >>
> >> Yes, this is expected. X86 implements a strongly ordered memory model,
> >> so a "store-to-memory" instruction ensures that the new value is
> >> immediately observed by other CPUs.
> >>
> >> FWIW the current code is wrong even on X86, because it does nothing to
> >> prevent compiler optimizations. The compiler is then allowed to reorder
> >> instructions so that the write to page_flag_buf->ready happens after
> >> other writes; with a bit of bad scheduling luck, the consumer thread
> >> may see an inconsistent state (e.g. read a stale page_flag_buf->pfn).
> >> Note that thanks to how compilers are designed (today), this issue is
> >> more or less hypothetical. Nevertheless, the use of atomics fixes it,
> >> because they also serve as memory barriers.
>
> Thank you Petr, for the information.  I was wondering whether atomic
> operations might be necessary for the other members of page_flag_buf,
> but it looks like they won't be necessary in this case.
>
> Then I was convinced that the issue would be fixed by removing the
> inconsistency of page_flag_buf->ready.  And the patch tested ok, so ack.
>

Thank you all for the patch review, patch testing and comments, these
have been so helpful!

Thanks,
Tao Liu

> Thanks,
> Kazu
>
> >
> > Thanks a lot for your detailed explanation, it's very helpful! I
> > haven't thought of the possibility of instruction reordering and
> > atomic_rw prevents the reorder.
> >
> > Thanks,
> > Tao Liu
> >
> >>
> >> Petr T
> >>