linux-kernel - Re: possible deadlock in aio

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Date:   Mon, 10 Sep 2018 20:14:20 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     syzbot <syzbot+5b1df0420c523b45a953@...kaller.appspotmail.com>,
        bcrl <bcrl@...ck.org>, linux-aio <linux-aio@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: possible deadlock in aio_poll

On Mon, Sep 10, 2018 at 6:53 PM, Christoph Hellwig <hch@...radead.org> wrote:
> On Mon, Sep 10, 2018 at 12:41:05AM -0700, syzbot wrote:
>> =====================================================
>> WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
>> 4.19.0-rc2+ #229 Not tainted
>> -----------------------------------------------------
>> syz-executor2/9399 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
>> 00000000126506e0 (&ctx->fd_wqh){+.+.}, at: spin_lock
>> include/linux/spinlock.h:329 [inline]
>> 00000000126506e0 (&ctx->fd_wqh){+.+.}, at: aio_poll+0x760/0x1420
>> fs/aio.c:1747
>>
>> and this task is already holding:
>> 000000002bed6bf6 (&(&ctx->ctx_lock)->rlock){..-.}, at: spin_lock_irq
>> include/linux/spinlock.h:354 [inline]
>> 000000002bed6bf6 (&(&ctx->ctx_lock)->rlock){..-.}, at: aio_poll+0x738/0x1420
>> fs/aio.c:1746
>> which would create a new lock dependency:
>>  (&(&ctx->ctx_lock)->rlock){..-.} -> (&ctx->fd_wqh){+.+.}
>
> ctx->fd_wqh seems to only exist in userfaultfd, which indeed seems
> to do strange open coded waitqueue locking, and seems to fail to disable
> irqs.  Something like this should fix it:

Why do pollable waitqueues need to disable interrupts generally?

I don't see anything fundamental in the poll interface to force this
requirement on users of that interface.

Thanks,
Miklos