linux-kernel - Re: possible deadlock in blkdev_reread

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Date:   Thu, 13 Sep 2018 14:58:54 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Eric Biggers <ebiggers3@...il.com>,
        syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@...kaller.appspotmail.com,
        Jens Axboe <axboe@...nel.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: possible deadlock in blkdev_reread_part

On Wed, May 2, 2018 at 1:23 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Wed, May 2, 2018 at 12:30 PM, Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> Dmitry Vyukov wrote:
>>> > syzbot is reporting various bugs which involve /dev/loopX.
>>> > Two of them
>>> >
>>> >   INFO: rcu detected stall in lo_ioctl
>>> >   https://syzkaller.appspot.com/bug?id=7b49fb610af9cca78c24e9f796f2e8b0d5573997
>>> >
>>> >   general protection fault in lo_ioctl (2)
>>> >   https://syzkaller.appspot.com/bug?id=f3cfe26e785d85f9ee259f385515291d21bd80a3
>>>
>>> /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>>
>>> Now there is a repro for this one. I've pushed it to kernel mailing lists:
>>>
>>> https://groups.google.com/d/msg/syzkaller-bugs/c8KUcTAzTvA/3o_7g6-tAwAJ
>>
>> OK, thanks. But among loop related reports, this will be a dup of
>> "INFO: rcu detected stall in blkdev_ioctl" which already has C reproducer.
>> Should we merge them?
>
> Yes, sure, I will take care of it.

1. I forgot to take care of it.

2. "INFO: rcu detected stall in blkdev_ioctl" was fixed 3 months ago:
https://syzkaller.appspot.com/bug?id=1f7b710f4110f225aed1f4263ec2b98b8dbd472e

but this bug still happens up until now:
https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889

So this is a different bug.



>>   INFO: rcu detected stall in blkdev_ioctl
>>   https://syzkaller.appspot.com/bug?id=1f7b710f4110f225aed1f4263ec2b98b8dbd472e
>>
>>   general protection fault in lo_ioctl (2)
>>   https://syzkaller.appspot.com/bug?id=f3cfe26e785d85f9ee259f385515291d21bd80a3
>>   #syz dup: INFO: rcu detected stall in blkdev_ioctl
>>
>>   INFO: rcu detected stall in lo_compat_ioctl
>>   https://syzkaller.appspot.com/bug?id=6299555c4e252b53f7a2ae2b8216cc9456c56ac0
>>   #syz dup: INFO: rcu detected stall in blkdev_ioctl
>>
>>   INFO: rcu detected stall in lo_ioctl
>>   https://syzkaller.appspot.com/bug?id=7b49fb610af9cca78c24e9f796f2e8b0d5573997
>>   #syz dup: INFO: rcu detected stall in blkdev_ioctl
>>
>>   INFO: task hung in lo_ioctl
>>   https://syzkaller.appspot.com/bug?id=608144371e7fc2cb6285b9ed871fb1eb817a61ce
>>
>>   INFO: task hung in lo_open (2)
>>   https://syzkaller.appspot.com/bug?id=1f93b57f496d969efb9fb24167f6f9de5ee068fd
>>
>>   possible deadlock in blkdev_reread_part
>>   https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889
>>
>>   INFO: task hung in loop_control_ioctl
>>   https://syzkaller.appspot.com/bug?id=61fe32c77ea00412c5149bd34649a65b7f672b5e
>>
>>   WARNING in sysfs_remove_group
>>   https://syzkaller.appspot.com/bug?id=3f86c0edf75c86d2633aeb9dd69eccc70bc7e90b
>>
>>>
>>> > suggest that loop module is not thread safe. The former suggests that
>>> > l->lo_backing_file is forming circular loop and the latter suggests that
>>> > l->lo_backing_file became NULL.