Re: [6.5-rc5 regression] core dump hangs (was Re: [Bug report] fstests generic/051 (on xfs) hang on latest linux v6.5-rc5+)

[Darrick J. Wong]

On Sun, Jun 11, 2023 at 08:14:25PM -0700, Linus Torvalds wrote:
> On Sun, Jun 11, 2023 at 7:22 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >
> > I guess the regression fix needs a regression fix....
> 
> Yup.
> 
> From the description of the problem, it sounds like this happens on
> real hardware, no vhost anywhere?
> 
> Or maybe Darrick (who doesn't see the issue) is running on raw
> hardware, and you and Zorro are running in a virtual environment?

Ahah, it turns out that liburing-dev isn't installed on the test fleet,
so fstests didn't get built with io_uring support.  That probably
explains why I don't see any of these hangs.

Oh.  I can't *install* the debian liburing-dev package because it has
a versioned dependency on linux-libc-dev >= 5.1, which isn't compatible
with me having a linux-libc-dev-djwong package that contains the uapi
headers for the latest upstream kernel and Replaces: linux-libc-dev.
So either I have to create a dummy linux-libc-dev with adequate version
number that pulls in my own libc header package, or rename that package.

<sigh> It's going to take me a while to research how best to split this
stupid knot.

--D

> It sounds like zap_other_threads() and coredump_task_exit() do not
> agree about the core_state->nr_threads counting, which is part of what
> changed there.
> 
> [ Goes off to look ]
> 
> Hmm. Both seem to be using the same test for
> 
>     (t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER
> 
> which I don't love - I don't think io_uring threads should participate
> in core dumping either, so I think the test could just be
> 
>     (t->flags & PF_IO_WORKER)
> 
> but that shouldn't be the issue here.
> 
> But according to
> 
>   https://lore.kernel.org/all/20230611124836.whfktwaumnefm5z5@zlang-mailbox/
> 
> it's clearly hanging in wait_for_completion_state() in
> coredump_wait(), so it really looks like some confusion about that
> core_waiters (aka core_state->nr_threads) count.
> 
> Oh. Humm. Mike changed that initial rough patch of mine, and I had
> moved the "if you don't participate in c ore dumps" test up also past
> the "do_coredump()" logic.
> 
> And I think it's horribly *wrong* for a thread that doesn't get
> counted for core-dumping to go into do_coredump(), because then it
> will set the "core_state" to possibly be the core-state of the vhost
> thread that isn't even counted.
> 
> So *maybe* this attached patch might fix it? I haven't thought very
> deeply about this, but vhost workers most definitely shouldn't call
> do_coredump(), since they are then not counted.
> 
> (And again, I think we should just check that PF_IO_WORKER bit, not
> use this more complex test, but that's a separate and bigger change).
> 
>               Linus

>  kernel/signal.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 2547fa73bde5..a1e11ee8537c 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2847,6 +2847,10 @@ bool get_signal(struct ksignal *ksig)
>  		 */
>  		current->flags |= PF_SIGNALED;
>  
> +		/* vhost workers don't participate in core dups */
> +		if ((current->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)
> +			goto out;
> +
>  		if (sig_kernel_coredump(signr)) {
>  			if (print_fatal_signals)
>  				print_fatal_signal(ksig->info.si_signo);