Re: possible deadlock in io_submit_one

[Eric Biggers]

Hi Bart and Christoph,

On Mon, Feb 04, 2019 at 06:03:04PM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    5eeb63359b1e Merge tag 'for-linus' of git://git.kernel.org..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17906f64c00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2e0064f906afee10
> dashboard link: https://syzkaller.appspot.com/bug?extid=a3accb352f9c22041cfa
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=156479f8c00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128c75c4c00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+a3accb352f9c22041cfa@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> =====================================================
> WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
> 5.0.0-rc4+ #56 Not tainted
> -----------------------------------------------------
> syz-executor263/8874 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> 00000000c469f622 (&ctx->fd_wqh){....}, at: spin_lock
> include/linux/spinlock.h:329 [inline]
> 00000000c469f622 (&ctx->fd_wqh){....}, at: aio_poll fs/aio.c:1772 [inline]
> 00000000c469f622 (&ctx->fd_wqh){....}, at: __io_submit_one fs/aio.c:1875
> [inline]
> 00000000c469f622 (&ctx->fd_wqh){....}, at: io_submit_one+0xedf/0x1cf0
> fs/aio.c:1908
> 
> and this task is already holding:
> 00000000829de875 (&(&ctx->ctx_lock)->rlock){..-.}, at: spin_lock_irq
> include/linux/spinlock.h:354 [inline]
> 00000000829de875 (&(&ctx->ctx_lock)->rlock){..-.}, at: aio_poll
> fs/aio.c:1771 [inline]
> 00000000829de875 (&(&ctx->ctx_lock)->rlock){..-.}, at: __io_submit_one
> fs/aio.c:1875 [inline]
> 00000000829de875 (&(&ctx->ctx_lock)->rlock){..-.}, at:
> io_submit_one+0xeb6/0x1cf0 fs/aio.c:1908
> which would create a new lock dependency:
>  (&(&ctx->ctx_lock)->rlock){..-.} -> (&ctx->fd_wqh){....}
> 

This is still happening.  See
https://syzkaller.appspot.com/text?tag=CrashReport&x=129eb971a00000 for a report
on Linus' tree from 5 days ago.

I see that a few months ago there was a commit

	commit d3d6a18d7d351cbcc9b33dbedf710e65f8ce1595
	Author: Bart Van Assche <bvanassche@xxxxxxx>
	Date:   Fri Feb 8 16:59:49 2019 -0800

	    aio: Fix locking in aio_poll()

but apparently it didn't fully fix the problem.

- Eric