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Date:   Thu, 24 Sep 2020 06:44:12 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     syzbot <syzbot+c32502fd255cb3a44048@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: possible deadlock in xfrm_policy_delete

On Thu, Sep 24, 2020 at 6:36 AM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Sun, Sep 20, 2020 at 01:22:14PM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    5fa35f24 Add linux-next specific files for 20200916
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1122e2d9900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=6bdb7e39caf48f53
> > dashboard link: https://syzkaller.appspot.com/bug?extid=c32502fd255cb3a44048
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+c32502fd255cb3a44048@...kaller.appspotmail.com
> >
> > =====================================================
> > WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
> > 5.9.0-rc5-next-20200916-syzkaller #0 Not tainted
> > -----------------------------------------------------
> > syz-executor.1/13775 [HC0[0]:SC0[4]:HE1:SE0] is trying to acquire:
> > ffff88805ee15a58 (&net->xfrm.xfrm_policy_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:359 [inline]
> > ffff88805ee15a58 (&net->xfrm.xfrm_policy_lock){+...}-{2:2}, at: xfrm_policy_delete+0x3a/0x90 net/xfrm/xfrm_policy.c:2236
> >
> > and this task is already holding:
> > ffff8880a811b1e0 (k-slock-AF_INET6){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:354 [inline]
> > ffff8880a811b1e0 (k-slock-AF_INET6){+.-.}-{2:2}, at: tcp_close+0x6e3/0x1200 net/ipv4/tcp.c:2503
> > which would create a new lock dependency:
> >  (k-slock-AF_INET6){+.-.}-{2:2} -> (&net->xfrm.xfrm_policy_lock){+...}-{2:2}
> >
> > but this new dependency connects a SOFTIRQ-irq-safe lock:
> >  (k-slock-AF_INET6){+.-.}-{2:2}
> >
> > ... which became SOFTIRQ-irq-safe at:
> >   lock_acquire+0x1f2/0xaa0 kernel/locking/lockdep.c:5398
> >   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
> >   _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
> >   spin_lock include/linux/spinlock.h:354 [inline]
> >   sctp_rcv+0xd96/0x2d50 net/sctp/input.c:231
>
> What's going on with all these bogus lockdep reports?
>
> These are two completely different locks, one is for TCP and the
> other is for SCTP.  Why is lockdep suddenly beoming confused about
> this?
>
> FWIW this flood of bogus reports started on 16/Sep.


FWIW one of the dups of this issue was bisected to:

commit 1909760f5fc3f123e47b4e24e0ccdc0fc8f3f106
Author: Ahmed S. Darwish <a.darwish@...utronix.de>
Date:   Fri Sep 4 15:32:31 2020 +0000

    seqlock: PREEMPT_RT: Do not starve seqlock_t writers

Can it be related?


A number of other new lockdep reports were bisected to the following
one, which was true intentional root cause of these, but it looks a
bit too old to cause the xfrm reports:

commit f08e3888574d490b31481eef6d84c61bedba7a47
Author: Boqun Feng <boqun.feng@...il.com>
Date: Fri Aug 7 07:42:30 2020 +0000

  lockdep: Fix recursive read lock related safe->unsafe detection

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