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

Gustavo Pimentel
2021-02-11T10:56:14Z

On Thu, Feb 11, 2021 at 10:33:25, Greg Kroah-Hartman
<gregkh@linuxfoundation.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@linuxfoundation.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@linuxfoundation.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