linux-kernel - Re: INFO: task hung in lo

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Date:   Sun, 20 Jan 2019 11:35:51 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Omar Sandoval <osandov@...ndov.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...com>,
        syzbot 
        <bot+abdba5bc6de135d7622f00756da97998425b6de5@...kaller.appspotmail.com>,
        Jens Axboe <axboe@...nel.dk>, Ming Lei <tom.leiming@...il.com>,
        Hannes Reinecke <hare@...e.de>, shli@...com,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Ingo Molnar <mingo@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>, knikanth@...e.de
Subject: Re: INFO: task hung in lo_ioctl

On 2019/01/20 3:56, Dmitry Vyukov wrote:
>> Seems easy enough to fix without resorting to __mutex_owner() (untested):
>>
>>
>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>> index 264abaaff662..cee258d12a1e 100644
>> --- a/drivers/block/loop.c
>> +++ b/drivers/block/loop.c
>> @@ -1300,12 +1300,13 @@ loop_get_status_old(struct loop_device *lo, struct loop_info __user *arg) {
>>  static int
>>  loop_get_status64(struct loop_device *lo, struct loop_info64 __user *arg) {
>>         struct loop_info64 info64;
>> -       int err = 0;
>> +       int err;
>>
>> -       if (!arg)
>> -               err = -EINVAL;
>> -       if (!err)
>> -               err = loop_get_status(lo, &info64);
>> +       if (!arg) {
>> +               mutex_unlock(&lo->lo_ctl_mutex);
>> +               return -EINVAL;
>> +       }
>> +       err = loop_get_status(lo, &info64);
>>         if (!err && copy_to_user(arg, &info64, sizeof(info64)))
>>                 err = -EFAULT;
>>
>>
>> I'll test it and send it up when I get into the office.
> 
> 
> Was this ever submitted? Or some other fix for this?
> 
> The bug is still open, but last happened 289 days ago:
> https://syzkaller.appspot.com/bug?id=608144371e7fc2cb6285b9ed871fb1eb817a61ce
> 
> But it also has 10 duplicates, some of which happened much more recently.
> If a fix was submitted, but Reported-by tag wasn't added this open bug
> can now mask lots of other new bugs.
> 

The commit for this specific patch is bdac616db9bbadb9 ("loop: fix LOOP_GET_STATUS
lock imbalance"). But the root cause of these hung tasks would be fixed by a series
containing commit 1dded9acf6dc9a34 ("Avoid circular locking dependency between
loop_ctl_mutex and bd_mutex") or commit 04906b2f542c2362 ("blockdev: Fix livelocks
on loop device"). We were not aware of these bugs when you marked these reports as
duplicates on 2017/12/12. You can undup them and fix them if you want.