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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Zorro Lang <zlang@redhat.com>, linux-xfs@vger.kernel.org,
        Mike Christie <michael.christie@oracle.com>,
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Date:   Mon, 12 Jun 2023 03:45:12 -0500
In-Reply-To: <CAHk-=wj0NuJaRNC4o6FVAJgKAFJ5HWcBV5VJw6RGV0ZahqOOZA@mail.gmail.com>
        (Linus Torvalds's message of "Sun, 11 Jun 2023 23:11:28 -0700")
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Subject: Re: [6.5-rc5 regression] core dump hangs (was Re: [Bug report]
 fstests generic/051 (on xfs) hang on latest linux v6.5-rc5+)
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Sun, Jun 11, 2023 at 10:49 PM Dave Chinner <david@fromorbit.com> wrote:
>>
>> On Sun, Jun 11, 2023 at 10:34:29PM -0700, Linus Torvalds wrote:
>> >
>> > So that "!=" should obviously have been a "==".
>>
>> Same as without the condition - all the fsstress tasks hang in
>> do_coredump().
>
> Ok, that at least makes sense. Your "it made things worse" made me go
> "What?" until I noticed the stupid backwards test.
>
> I'm not seeing anything else that looks odd in that commit
> f9010dbdce91 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps
> regression").
>
> Let's see if somebody else goes "Ahh" when they wake up tomorrow...

It feels like there have been about half a dozen bugs pointed out in
that version of the patch.  I am going to have to sleep before I can get
as far as "Ahh"

One thing that really stands out for me is.

if (test_if_loop_should_continue) {
	set_current_state(TASK_INTERRUPTIBLE);
        schedule();
}

/* elsewhere */
llist_add(...);
wake_up_process()

So it is possible that the code can sleep indefinitely waiting for a
wake-up that has already come, because the order of set_current_state
and the test are in the wrong order.

Unfortunately I don't see what would effect a coredump on a process that
does not trigger the vhost_worker code.


About the only thing I can image is if io_uring is involved.  Some of
the PF_IO_WORKER code was changed, and the test
"((t->flags & (PF_USER_WORKER | PF_IO_WORKER)) != PF_USER_WORKER)"
was sprinkled around.

That is the only code outside of vhost specific code that was changed.


Is io_uring involved in the cases that hang?


Eric