Re: possible deadlock in send_sigio

[Waiman Long]

On 4/4/20 1:55 AM, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    bef7b2a7 Merge tag 'devicetree-for-5.7' of git://git.kerne..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15f39c5de00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=91b674b8f0368e69
> dashboard link: https://syzkaller.appspot.com/bug?extid=a9fb1457d720a55d6dc5
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1454c3b7e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12a22ac7e00000
>
> The bug was bisected to:
>
> commit 7bc3e6e55acf065500a24621f3b313e7e5998acf
> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Date:   Thu Feb 20 00:22:26 2020 +0000
>
>      proc: Use a list of inodes to flush from proc
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=165c4acde00000
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=155c4acde00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=115c4acde00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+a9fb1457d720a55d6dc5@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
>
> ========================================================
> WARNING: possible irq lock inversion dependency detected
> 5.6.0-syzkaller #0 Not tainted
> --------------------------------------------------------
> ksoftirqd/0/9 just changed the state of lock:
> ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigio+0xa9/0x340 fs/fcntl.c:800
> but this lock took another, SOFTIRQ-unsafe lock in the past:
>   (&pid->wait_pidfd){+.+.}-{2:2}
>
>
> and interrupts could create inverse lock ordering between them.
>
>
> other info that might help us debug this:
>   Possible interrupt unsafe locking scenario:
>
>         CPU0                    CPU1
>         ----                    ----
>    lock(&pid->wait_pidfd);
>                                 local_irq_disable();
>                                 lock(tasklist_lock);
>                                 lock(&pid->wait_pidfd);
>    <Interrupt>
>      lock(tasklist_lock);
>
>   *** DEADLOCK ***

That is a false positive. The qrwlock has the special property that it 
becomes unfair (for read lock) at interrupt context. So unless it is 
taking a write lock in the interrupt context, it won't go into deadlock. 
The current lockdep code does not capture the full semantics of qrwlock 
leading to this false positive.

Cheers,
Longman