linux-kernel - Re: [PATCH] staging: android: Remove set but unused variable in ashmem.c

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Date:   Sat, 24 Jul 2021 09:49:35 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        Hridya Valsaraju <hridya@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-staging@...ts.linux.dev
Subject: Re: [PATCH] staging: android: Remove set but unused variable in ashmem.c

On Friday, July 23, 2021 10:14:46 PM CEST Suren Baghdasaryan wrote:
> On Fri, Jul 23, 2021 at 1:05 PM Fabio M. De Francesco
> 
> <fmdefrancesco@...il.com> wrote:
> > Remove variable 'inode' tnat is set but unused. Issue detected
> > by building with warning option -Wunused-but-set-variable.
> > 
> > [...]
> > 
> > -               inode = file_inode(vmfile);
> > 
> >                 lockdep_set_class(&inode->i_rwsem, 
&backing_shmem_inode_class);
> 
> How about its usage in the above lockdep_set_class(&inode->i_rwsem,
> ...) call? 

I'm sorry. I didn't notice that 'inode' is used soon after assignment, just in 
the line that follows it. I was also too confident that rebuilding the driver 
would have triggered an error if it is used somewhere else. I was obviously 
wrong.

Furthermore, I usually automatically search with grep or other tools for other 
occurrences of a symbol in the function and the file, but this time I must 
have forgotten to do that.

> I'm guessing you are building with CONFIG_LOCKDEP=n.

Actually my .config sets CONFIG_LOCKDEP_SUPPORT=y. Did you refer to this? 
There is no CONFIG_LOCKDEP in the file.

I've just run "make clean && make C=2 -j8 drivers/staging/android/ W=1" one 
more time and it still builds.  How is it possible that the code builds 
correctly with my configuration? 

This is the output (the last two lines) of 'make':

CC      drivers/staging/android/ashmem.o
AR      drivers/staging/android/built-in.

There are no errors or warnings, even with that "lockdep_set_class(&inode-
>i_rwsem, &backing_shmem_inode_class);" left as-is after 'inode' removal. 

I really can't understand what I'm missing.

> Have you tried adding __maybe_unused in inode variable definition to
> get rid of the warning?
> 
Actually, I didn't know that directive that you mention. I'll read the 
documentation and, if and where suited, I'll use it.

Thanks,

Fabio
>
> > [...]
> > --
> > 2.32.0