linux-kernel - Re: [RFC PATCH] spi: Take the SPI IO-mutex in the spi

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Date:   Tue, 17 Nov 2020 14:28:23 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] spi: Take the SPI IO-mutex in the spi_setup() method

On Tue, Nov 17, 2020 at 12:56:44PM +0200, Andy Shevchenko wrote:
> On Tue, Nov 17, 2020 at 11:45 AM Serge Semin
> <Sergey.Semin@...kalelectronics.ru> wrote:
> >
> > I've discovered that due to the recent commit 49d7d695ca4b ("spi: dw:
> > Explicitly de-assert CS on SPI transfer completion") a concurrent usage of
> > the spidev devices with different chip-selects causes the "SPI transfer
> > timed out" error.
> 
> I'll read this later...
> 
> > +       mutex_lock(&spi->controller->io_mutex);
> > +
> >         if (spi->controller->setup)
> >                 status = spi->controller->setup(spi);
> >
> >         if (spi->controller->auto_runtime_pm && spi->controller->set_cs) {
> >                 status = pm_runtime_get_sync(spi->controller->dev.parent);
> 

> I didn't check what this lock is protecting,

It is used to protect the SPI io operations. So it's locked only
during the SPI memory operations and the SPI-message execution. That's
the time when the core toggles the controller chip-selects by calling
the spi_set_cs() method and the set_cs callback.

> but have you checked all
> PM runtime callbacks if they are not taking the lock. When you call PM
> runtime functions with 'sync' it may include a lot of work, some of
> which may sleep (not a problem for mutex) and may take arbitrary locks
> (might be a deadlock in case of trying the same lock).

Yeah, I understand that. Simple grepping hasn't showed anyone else but
the SPI-core using it. So unless the controllers PM methods also call
spi_setup() or request SPI-transfers, there shouldn't be a deadlock.
Moreover as I can see from the __spi_pump_messages() method the
IO-mutex is locked during the sync-suffixed PM-methods invocation.
AFAICS locking io_mutex around the PM-methods here shouldn't cause
problems. But of course testing it in various platforms/controllers is
always welcome.

-Sergey

> 
> >                 if (status < 0) {
> > +                       mutex_unlock(&spi->controller->io_mutex);
> >                         pm_runtime_put_noidle(spi->controller->dev.parent);
> >                         dev_err(&spi->controller->dev, "Failed to power device: %d\n",
> >                                 status);
> > @@ -3354,6 +3357,8 @@ int spi_setup(struct spi_device *spi)
> >                 spi_set_cs(spi, false);
> >         }
> >
> > +       mutex_unlock(&spi->controller->io_mutex);
> > +
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko