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SubjectRe: INFO: task hung in fuse_reverse_inval_entry
On Mon, Jul 23, 2018 at 2:22 PM, Dmitry Vyukov <> wrote:
> On Mon, Jul 23, 2018 at 2:12 PM, Miklos Szeredi <> wrote:
>> On Mon, Jul 23, 2018 at 10:11 AM, Dmitry Vyukov <> wrote:
>>> On Mon, Jul 23, 2018 at 9:59 AM, syzbot
>>> <> wrote:
>>>> Hello,
>>>> syzbot found the following crash on:
>>>> HEAD commit: d72e90f33aa4 Linux 4.18-rc6
>>>> git tree: upstream
>>>> console output:
>>>> kernel config:
>>>> dashboard link:
>>>> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
>>>> syzkaller repro:
>>>> C reproducer:
>>> Hi fuse maintainers,
>>> We are seeing a bunch of such deadlocks in fuse on syzbot. As far as I
>>> understand this is mostly working-as-intended (parts about deadlocks
>>> in Documentation/filesystems/fuse.txt). The intended way to resolve
>>> this is aborting connections via fusectl, right?
>> Yes. Alternative is with "umount -f".
>>> The doc says "Under
>>> the fuse control filesystem each connection has a directory named by a
>>> unique number". The question is: if I start a process and this process
>>> can mount fuse, how do I kill it? I mean: totally and certainly get
>>> rid of it right away? How do I find these unique numbers for the
>>> mounts it created?
>> It is the device number found in st_dev for the mount. Other than
>> doing stat(2) it is possible to find out the device number by reading
>> /proc/$PID/mountinfo (third field).
> Thanks. I will try to figure out fusectl connection numbers and see if
> it's possible to integrate aborting into syzkaller.
>>> Taking into account that there is usually no
>>> operator attached to each server, I wonder if kernel could somehow
>>> auto-abort fuse on kill?
>> Depends on what the fuse server is sleeping on. If it's trying to
>> acquire an inode lock (e.g. unlink(2)), which is classical way to
>> deadlock a fuse filesystem, then it will go into an uninterruptible
>> sleep. There's no way in which that process can be killed except to
>> force a release of the offending lock, which can only be done by
>> aborting the request that is being performed while holding that lock.
> I understand that it is not killed today, but I am asking if we can
> make it killable. It's all code that we can change, and if a human
> operator can do it, it can be done pure programmatically on kill too,
> right?

Hmm, you mean if a process is in an uninterruptible sleep trying to
acquire a lock on a fuse filesystem and is killed, then the fuse
filesystem should be aborted?

Even if we'd manage to implement that, it's a large backward
incompatibility risk.

I don't argue that it can be done, but I would definitely argue *if*
it should be done.


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