Re: rcu_sync_dtor() warning question

[Oleg Nesterov <oleg@redhat.com>]

Sorry for late reply.

You do not need my help, I know nothing about fs ;)  but just in case...

On 07/26, Mateusz Guzik wrote:
>
> Welp.
>
> syzbot did the bisect, it's not any of the above, instead:
>
> commit b62e71be2110d8b52bf5faf3c3ed7ca1a0c113a5
> Author: Chao Yu <chao@kernel.org>
> Date:   Sun Apr 23 15:49:15 2023 +0000
>
>     f2fs: support errors=remount-ro|continue|panic mountoption
>
> https://lore.kernel.org/linux-fsdevel/0000000000004ff2dc061e281637@google.com/T/#m90c03813e12e5cdff1eeada8f9ab581d5f039c76
>
> That said, the stuff I mentioned still looks highly suspicious so I
> have to something to investigate regardless.

Did you see the patch from Hillf ?
https://lore.kernel.org/all/20240727011616.2144-1-hdanton@sina.com/
it seems to fix the problem...

Of course I don't understand this patch, but afaics SB_RDONLY can confuse
thaw_super_locked(). If sb_rdonly() is true, thaw_super_locked() assumes
that freeze_super() didn't call sb_wait_write() -> percpu_down_write().

So in this case thaw_super_locked() just clears sb->s_writers.frozen and
goes to the "out_deactivate" label bypassing sb_freeze_unlock().

Oleg.