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Date:   Mon, 1 Mar 2021 18:23:57 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        syzbot <syzbot+506c8a2a115201881d45@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Eric Dumazet <edumazet@google.com>,
        Mina Almasry <almasrymina@google.com>
Subject: Re: possible deadlock in sk_clone_lock
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On Mon 01-03-21 08:39:22, Shakeel Butt wrote:
> On Mon, Mar 1, 2021 at 7:57 AM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > Then how come this can ever be a problem? in_task() should exclude soft
> > irq context unless I am mistaken.
> >
> 
> If I take the following example of syzbot's deadlock scenario then
> CPU1 is the one freeing the hugetlb pages. It is in the process
> context but has disabled softirqs (see __tcp_close()).
> 
>         CPU0                    CPU1
>         ----                    ----
>    lock(hugetlb_lock);
>                                 local_irq_disable();
>                                 lock(slock-AF_INET);
>                                 lock(hugetlb_lock);
>    <Interrupt>
>      lock(slock-AF_INET);
> 
> So, this deadlock scenario is very much possible.

OK, I see the point now. I was focusing on the IRQ context and hugetlb
side too much. We do not need to be freeing from there. All it takes is
to get a dependency chain over a common lock held here. Thanks for
bearing with me.

Let's see whether we can make hugetlb_lock irq safe.

-- 
Michal Hocko
SUSE Labs