linux-kernel - Re: [Question: devm_kfree] When should devm

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Message-ID: <20190528071400.GB18498@zhanggen-UX430UQ>
Date:   Tue, 28 May 2019 15:14:00 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     rafael@...nel.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Question: devm_kfree] When should devm_kfree() be used?

On Tue, May 28, 2019 at 08:49:49AM +0200, Greg KH wrote:
> On Tue, May 28, 2019 at 08:32:57AM +0800, Gen Zhang wrote:
> > devm_kmalloc() is used to allocate memory for a driver dev. Comments
> > above the definition and doc 
> > (https://www.kernel.org/doc/Documentation/driver-model/devres.txt) all
> > imply that allocated the memory is automatically freed on driver attach,
> > no matter allocation fail or not. However, I examined the code, and
> > there are many sites that devm_kfree() is used to free devm_kmalloc().
> > e.g. hisi_sas_debugfs_init() in drivers/scsi/hisi_sas/hisi_sas_main.c.
> > So I am totally confused about this issue. Can anybody give me some
> > guidance? When should we use devm_kfree()?
> 
> If you "know" you need to free the memory now, call devm_kfree().  If
> you want to wait for it to be cleaned up latter, like normal, then do
> not call it.
> 
> Do you have a driver that you think uses it incorrectly?
> 
> thanks,
> 
> greg k-h
Yes, that is my question! I do have several patches about devm_kfree() 
and debate with maintainers. e.g. you can kindly refer to this lkml:
https://lkml.org/lkml/2019/5/23/1547. 

In tegra_wm9712_driver_probe(), 'machine->codec' is allocated by
platform_device_alloc(). When it is NULL, function returns ENOMEM.
However, 'machine' is allocated by devm_kzalloc() before this site.
Thus we should free 'machine' before function ends to prevent memory
leaking.

And as mentioned above, e.g. hisi_sas_debugfs_init() in 
drivers/scsi/hisi_sas/hisi_sas_main.c, devm_kfree() is used to free 
devm_kmalloc().

So, thanks for looking into this, Greg.

Thanks
Gen