linux-kernel - Re: [PATCH v2 0/5] IB/hfi1: Remove write() and use ioctl() for user access

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Date:	Thu, 12 May 2016 13:25:08 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Dennis Dalessandro <dennis.dalessandro@...el.com>
Cc:	dledford@...hat.com, linux-rdma@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] IB/hfi1: Remove write() and use ioctl() for user
 access

On Thu, May 12, 2016 at 03:07:38PM -0400, Dennis Dalessandro wrote:
> >>There is also a driver software version being exported via a sysfs
> >>file. This is needed so that user space applications (psm) can
> >>determine if it needs to do ioctl() or write().
> >
> >Why? Don't do this, just call ioctl() and if it fails then use write().
> 
> Is it really that big of a deal to export a version number?

If it isn't needed, don't add it..

> >another reference counted structure. You need to consider how all this
> >works when the driver is removed while the cdev is still open (or
> >driver remove is racing with the cdev release).
> 
> The driver can't be removed while the cdev is still open. I tested with a
> test code that opens /dev/hfi1_0 and spins. The use count as reported by
> lsmod ticks up and the driver can not be unloaded until I ctrl+c the test
> program.

Drivers can be removed in other ways, eg pci hot unplug. Do not assume
module_exit is the only way and rely on module ref counting for
correctness.

Jason