Re: possible deadlock in sk_clone_lock

[Shakeel Butt <shakeelb@google.com>]

On Mon, Mar 1, 2021 at 4:12 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 26-02-21 16:00:30, Shakeel Butt wrote:
> > On Fri, Feb 26, 2021 at 3:14 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> > >
> > > Cc: Michal
> > >
> > > On 2/26/21 2:44 PM, Shakeel Butt wrote:
> > > > On Fri, Feb 26, 2021 at 2:09 PM syzbot
> > > > <syzbot+506c8a2a115201881d45@syzkaller.appspotmail.com> wrote:
> > > <snip>
> > > >> other info that might help us debug this:
> > > >>
> > > >>  Possible interrupt unsafe locking scenario:
> > > >>
> > > >>        CPU0                    CPU1
> > > >>        ----                    ----
> > > >>   lock(hugetlb_lock);
> > > >>                                local_irq_disable();
> > > >>                                lock(slock-AF_INET);
> > > >>                                lock(hugetlb_lock);
> > > >>   <Interrupt>
> > > >>     lock(slock-AF_INET);
> > > >>
> > > >>  *** DEADLOCK ***
> > > >
> > > > This has been reproduced on 4.19 stable kernel as well [1] and there
> > > > is a reproducer as well.
> > > >
> > > > It seems like sendmsg(MSG_ZEROCOPY) from a buffer backed by hugetlb. I
> > > > wonder if we just need to make hugetlb_lock softirq-safe.
> > > >
> > > > [1] https://syzkaller.appspot.com/bug?extid=6383ce4b0b8ec575ad93
> > >
> > > Thanks Shakeel,
> > >
> > > Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task
> > > context") attempted to address this issue.  It uses a work queue to
> > > acquire hugetlb_lock if the caller is !in_task().
> > >
> > > In another recent thread, there was the suggestion to change the
> > > !in_task to in_atomic.
> > >
> > > I need to do some research on the subtle differences between in_task,
> > > in_atomic, etc.  TBH, I 'thought' !in_task would prevent the issue
> > > reported here.  But, that obviously is not the case.
> >
> > I think the freeing is happening in the process context in this report
> > but it is creating the lock chain from softirq-safe slock to
> > irq-unsafe hugetlb_lock. So, two solutions I can think of are: (1)
> > always defer the freeing of hugetlb pages to a work queue or (2) make
> > hugetlb_lock softirq-safe.
>
> There is __do_softirq so this should be in the soft IRQ context no?
> Is this really reproducible with kernels which have c77c0a8ac4c5
> applied?

Yes this is softirq context and syzbot has reproduced this on
linux-next 20210224.

>
> Btw. making hugetlb lock irq safe has been already discussed and it
> seems to be much harder than expected as some heavy operations are done
> under the lock. This is really bad.

What about just softirq-safe i.e. spin_[un]lock_bh()? Will it still be that bad?

> Postponing the whole freeing
> operation into a worker context is certainly possible but I would
> consider it rather unfortunate. We would have to add some sync mechanism
> to wait for hugetlb pages in flight to prevent from external
> observability to the userspace. E.g. when shrinking the pool.

I think in practice recycling of hugetlb pages is a rare event, so we
might get away without the sync mechanism. How about start postponing
the freeing without sync mechanism and add it later if there are any
user reports complaining?

> --
> Michal Hocko
> SUSE Labs