Re: [syzbot] possible deadlock in del_gendisk

From: Pavel Tatashin <pasha.tatashin@soleen.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>,
	Petr Vorel <pvorel@suse.cz>, Christoph Hellwig <hch@lst.de>,
	Jens Axboe <axboe@kernel.dk>,
	Stefan Haberland <sth@linux.ibm.com>,
	Jan Hoeppner <hoeppner@linux.ibm.com>,
	linux-block <linux-block@vger.kernel.org>,
	syzbot <syzbot+61e04e51b7ac86930589@syzkaller.appspotmail.com>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [syzbot] possible deadlock in del_gendisk
Date: Fri, 11 Jun 2021 11:18:50 -0400	[thread overview]
Message-ID: <CA+CK2bBe5muuGbHgfK7JjbzRE5ogf1oeD1iYeY6eJB046p9_ZQ@mail.gmail.com> (raw)
In-Reply-To: <CA+CK2bDWb2=bsoacY-eqZExObBpXuZE0a3Mr18_FXmGZTC5GnQ@mail.gmail.com>

On Fri, Jun 11, 2021 at 11:11 AM Pavel Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On Fri, Jun 11, 2021 at 10:47 AM Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
> >
> > On 2021/06/10 1:46, Tyler Hicks wrote:
> > > Thanks for doing this. I haven't had a chance to retry this commit with
> > > lockdep but I did re-review it and didn't think that it would be the
> > > cause of this lockdep report since it strictly reduced the usage of the
> > > loop_ctl_mutex.
> >
> > Well, I made commit 310ca162d779efee ("block/loop: Use global lock for ioctl() operation.")
> > because per device lock was not sufficient. Did commit 6cc8e7430801fa23 ("loop: scale loop
> > device by introducing per device lock") take this problem into account?
>
> This was my intention when I wrote 6cc8e7430801fa23 ("loop: scale loop
> device by introducing per device lock"). This is why this change does
> not simply revert 310ca162d779efee ("block/loop: Use global lock for
> ioctl() operation."), but keeps loop_ctl_mutex to protect the global
> accesses.  loop_control_ioctl() is still locked by global
> loop_ctl_mutex.

From commit log of:
 310ca162d779efee ("block/loop: Use global lock for ioctl() operation.")

    Since ioctl() request on loop devices is not frequent operation, we don't
    need fine grained locking. Let's use global lock in order to allow safe
    traversal at loop_validate_file().

Unfortunately using a global lock is not scalable here. At Microsoft
we had a slow startup issue because we have several loop devices per
every container. Also, this report:
https://lore.kernel.org/lkml/20210217145629.GA4503@xsang-OptiPlex-9020/

Pasha