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

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Date:   Thu, 11 Feb 2021 10:51:56 +0000
From:   Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 10:33:25, Greg Kroah-Hartman 
<gregkh@...uxfoundation.org> wrote:

> On Thu, Feb 11, 2021 at 10:21:07AM +0000, Gustavo Pimentel wrote:
> > On Thu, Feb 11, 2021 at 9:59:26, Greg Kroah-Hartman 
> > <gregkh@...uxfoundation.org> wrote:
> > 
> > > 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?
> > 
> > The write_store() does a procedure to enable the traffic on the write 
> > direction, however, the write_show() does a different procedure to 
> > calculate the link throughput speed, which uses a different set of 
> > registers on the HW.
> > 
> > Similar happens on the read_store() (which enable the traffic on the read 
> > direction) and on the read_show()
> > 
> > To summarize write_store() follows the same approach of read_store() and 
> > the write_show() of the read_show(). I added the mutex on those functions 
> > for instance to avoid while during the write_show() call the possibility 
> > of been called the read_show() messing up the link throughput speed 
> > calculation.
> > Or while during the write_store() call to be called the read_store or 
> > even the write_show() for the same reasons.
> 
> If you need to protect these types of things, but the lock down in the
> function that does this, not above it which forces people to audit
> everything and manually try to determine what lock is doing what for
> what.
> 
> Make it impossible to get wrong, as it is, you have to do extra work
> here to keep things working properly, always a bad idea in an api.

I think I understood what you mean, I will *reduce* the mutex scope to 
the basic functions that are called by the sysfs *_store() and *_show().

-Gustavo

> 
> thanks,
> 
> greg k-h