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 15:53:04 -0400
From:	Dennis Dalessandro <dennis.dalessandro@...el.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.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 01:25:08PM -0600, Jason Gunthorpe wrote:
>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..

For the reason I gave, I think it is needed so unless you are vehemently 
opposed to it I would prefer to leave 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.

Point taken. I'll look into this. So are you perhaps suggesting we do 
something like is done for uverbs_dev in ib_verbs_add_one() where there is a 
kobj for uverbs_dev and the parent of uverbs_dev->cdeb is set to that? In 
our case it would probably be something like hfi1_devdata.

-Denny