linux-kernel - Re: [PATCH v5 1/6] misc: Add Synopsys DesignWare xData IP driver

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Date:   Thu, 11 Feb 2021 10:59:26 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>
Cc:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Derek Kiernan <derek.kiernan@...inx.com>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v5 1/6] misc: Add Synopsys DesignWare xData IP driver

On Thu, Feb 11, 2021 at 09:50:33AM +0000, Gustavo Pimentel wrote:
> On Thu, Feb 11, 2021 at 9:30:16, Greg Kroah-Hartman 
> <gregkh@...uxfoundation.org> wrote:
> 
> > On Thu, Feb 11, 2021 at 10:08:38AM +0100, Gustavo Pimentel wrote:
> > > +static ssize_t write_show(struct device *dev, struct device_attribute *attr,
> > > +			  char *buf)
> > > +{
> > > +	struct pci_dev *pdev = to_pci_dev(dev);
> > > +	struct dw_xdata *dw = pci_get_drvdata(pdev);
> > > +	u64 rate;
> > > +
> > > +	mutex_lock(&dw->mutex);
> > > +	dw_xdata_perf(dw, &rate, true);
> > > +	mutex_unlock(&dw->mutex);
> > > +
> > > +	return sysfs_emit(buf, "%llu MB/s\n", rate);
> > 
> > Do not put units in a sysfs file, that should be in the documentation,
> > otherwise this forces userspace to "parse" the units which is a mess.
> 
> Okay.
> 
> > 
> > Same for the other sysfs file.
> > 
> > And why do you need a lock for this show function?
> 
> Maybe I understood it wrongly, please correct me in that case. The 
> dw_xdata_perf() is called on the write_show() and read_show(), to avoid a 
> possible race condition between those calls, I have added this mutex.

What race?  If the value changes with a write right after a read, what
does it matter?

What exactly are you trying to protect with this lock?

thanks,

greg k-h