From mboxrd@z Thu Jan  1 00:00:00 1970
From: hle@owl.eu.com (Hugo Lefeuvre)
Date: Tue, 5 Jun 2018 21:48:23 -0400
Subject: develoment workflow: how to avoid duplicate work ?
In-Reply-To: <20180605174713.GS14048@gavran.carpriv.carnet.hr>
References: <20180531014432.GA3722@hle-laptop.local>
 <20180603192336.GV14048@gavran.carpriv.carnet.hr>
 <20180603222556.GB20060@hle-laptop.local>
 <20180604033303.GX14048@gavran.carpriv.carnet.hr>
 <12368.1528128607@turing-police.cc.vt.edu>
 <20180604223131.GA1981@hle-laptop.local>
 <68638.1528178148@turing-police.cc.vt.edu>
 <20180605142016.GA2334@hle-laptop.local>
 <179873.1528209201@turing-police.cc.vt.edu>
 <20180605174713.GS14048@gavran.carpriv.carnet.hr>
Message-ID: <20180606014822.GA2933@hle-laptop.local>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

> > It's not duplication, it's increment/decrement of a counter.
> > 
> > Look for functions  with 'get' and 'put' in their names.
> 
> AFAICT counter is f_count in struct file, updated by fget and fput.

Yes, this is the counter I was speaking about. But to me the only way to
increment/decrement this counter was either to duplicate an existing
reference using dup/dup2, or to fork the process. This was wrong, and
I'm glad I learned that.

FTR: After taking a look at the kernel code, the ioctl syscall is
implemented in ksys_ioctl[0], which calls fdget/fdput to get the struct
fd, incrementing and decrementing the count.

Thanks your help !

> > I haven't looked at the code - there's an outside chance that the driver
> > isn't doing reference counting correctly.
> 
> pi433 driver does not mess with struct file too much so I guess it should
> be safe :)

I'll take a closer look at the TODO and come back later with a patch. If
there's nothing to do I'll remove it, otherwise I'll fix it.

Cheers,
 Hugo

[0] https://elixir.bootlin.com/linux/latest/source/fs/ioctl.c#L692

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             Hugo Lefeuvre (hle)    |    www.owl.eu.com
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