On Thu, Feb 11, 2021 at 9:59:26, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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.
This is the reason why I added those mutexes, maybe this isn't necessary
and it's overkill. Please advise me if a different approach can be done.
-Gustavo
>
> thanks,
>
> greg k-h
