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Date:   Sun, 3 Apr 2022 21:58:06 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     syzbot <syzbot+4f1a237abaf14719db49@syzkaller.appspotmail.com>,
        axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        Jan Kara <jack@suse.cz>
Subject: Re: [syzbot] INFO: task can't die in blkdev_common_ioctl
Message-ID: <Ykp63pk7aSj/bNRT@infradead.org>
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 <d3213c1a-f82a-02f6-b5b2-0d3242e0242a@I-love.SAKURA.ne.jp>
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On Sat, Apr 02, 2022 at 08:26:23PM +0900, Tetsuo Handa wrote:
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=168d578db00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=be9183de0824e4d7
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4f1a237abaf14719db49
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > 
> > Unfortunately, I don't have any reproducer for this issue yet.
> 
> This is a known problem, and Christoph does not like a mitigation patch.
> https://lkml.kernel.org/r/YgoQBTzb3b0l1SzP@infradead.org

And this report shows why: your patch makes the lock acquisition in
blkdev_fallocate killable.  Which would not help with this problem at
all, as it does not come through blkdev_fallocate.  The problem is that
the loop writing actual zeroes to the disk can take forever, and we
hold the invalidate_lock over that.