Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases

From: Jens Axboe <axboe@kernel.dk>
To: Jason Wang <jasowang@redhat.com>, Yongji Xie <xieyongji@bytedance.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Stefano Garzarella <sgarzare@redhat.com>,
	Parav Pandit <parav@nvidia.com>, Bob Liu <bob.liu@oracle.com>,
	Christoph Hellwig <hch@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	viro@zeniv.linux.org.uk, bcrl@kvack.org,
	Jonathan Corbet <corbet@lwn.net>,
	virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, kvm@vger.kernel.org, linux-aio@kvack.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases
Date: Wed, 27 Jan 2021 20:08:19 -0700	[thread overview]
Message-ID: <4e482f00-163a-f957-4665-141502cf4dff@kernel.dk> (raw)
In-Reply-To: <b5c9f2d4-5b95-4552-3886-f5cbcb7de232@redhat.com>

On 1/27/21 8:04 PM, Jason Wang wrote:
> 
> On 2021/1/27 下午5:11, Yongji Xie wrote:
>> On Wed, Jan 27, 2021 at 11:38 AM Jason Wang <jasowang@redhat.com> wrote:
>>>
>>> On 2021/1/20 下午2:52, Yongji Xie wrote:
>>>> On Wed, Jan 20, 2021 at 12:24 PM Jason Wang <jasowang@redhat.com> wrote:
>>>>> On 2021/1/19 下午12:59, Xie Yongji wrote:
>>>>>> Now we have a global percpu counter to limit the recursion depth
>>>>>> of eventfd_signal(). This can avoid deadlock or stack overflow.
>>>>>> But in stack overflow case, it should be OK to increase the
>>>>>> recursion depth if needed. So we add a percpu counter in eventfd_ctx
>>>>>> to limit the recursion depth for deadlock case. Then it could be
>>>>>> fine to increase the global percpu counter later.
>>>>> I wonder whether or not it's worth to introduce percpu for each eventfd.
>>>>>
>>>>> How about simply check if eventfd_signal_count() is greater than 2?
>>>>>
>>>> It can't avoid deadlock in this way.
>>>
>>> I may miss something but the count is to avoid recursive eventfd call.
>>> So for VDUSE what we suffers is e.g the interrupt injection path:
>>>
>>> userspace write IRQFD -> vq->cb() -> another IRQFD.
>>>
>>> It looks like increasing EVENTFD_WAKEUP_DEPTH should be sufficient?
>>>
>> Actually I mean the deadlock described in commit f0b493e ("io_uring:
>> prevent potential eventfd recursion on poll"). It can break this bug
>> fix if we just increase EVENTFD_WAKEUP_DEPTH.
> 
> 
> Ok, so can wait do something similar in that commit? (using async stuffs 
> like wq).

io_uring should be fine in current kernels, but aio would still be
affected by this. But just in terms of recursion, bumping it one more
should probably still be fine.

-- 
Jens Axboe