linux-kernel - Re: [PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release

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Date:   Wed, 13 Apr 2022 11:43:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Yao Hongbo <yaohongbo@...ux.alibaba.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        alikernel-developer@...ux.alibaba.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release

On Wed, Apr 13, 2022 at 05:25:40PM +0800, Yao Hongbo wrote:
> 
> 在 2022/4/13 下午4:51, Michael S. Tsirkin 写道:
> > On Wed, Apr 13, 2022 at 09:33:17AM +0200, Greg KH wrote:
> > > On Wed, Apr 13, 2022 at 03:01:42PM +0800, Yao Hongbo wrote:
> > > > If two userspace programs both open the PCI UIO fd, when one
> > > > of the program exits uncleanly, the other will cause IO hang
> > > > due to bus-mastering disabled.
> > > > 
> > > > It's a common usage for spdk/dpdk to use UIO. So, introduce refcnt
> > > > to avoid such problems.
> > > Why do you have multiple userspace programs opening the same device?
> > > Shouldn't they coordinate?
> > Or to restate, I think the question is, why not open the device
> > once and pass the FD around?
> 
> Hmm, it will have the same result, no matter  whether opening the same
> device or pass the FD around.

How?  You only open once, and close once.  Where is the multiple closes?

> Our expectation is that even if the primary process exits abnormally,  the
> second process can still send
> 
> or receive data.

Then use the same file descriptor.

> The impact of disabling pci bus-master is relatively large, and we should
> make some restrictions on
> this behavior.

Why?  UIO is "you better really really know what you are doing to use
this interface", right?  Just duplicate the fd and pass it around if you
must have multiple accesses to the same device.

And again, this will be a functional change.  How can you handle your
userspace on older kernels if you make this change?

thanks,

greg k-h