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Message-ID: <CAOCi2DF=KQ9c4Trd_C_L8DsyeBeJehh-TCpOswgVWwsNoyz+Bg@mail.gmail.com>
Date: Sun, 19 Mar 2017 18:46:14 +0530
From: Gargi Sharma <gs051095@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Lars-Peter Clausen <lars@...afoo.de>,
simran singhal <singhalsimran0@...il.com>,
Michael Hennerich <Michael.Hennerich@...log.com>,
Hartmut Knaack <knaack.h@....de>,
Pete Meerwald-Stadler <pmeerw@...erw.net>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
linux-kernel@...r.kernel.org, outreachy-kernel@...glegroups.com
Subject: Re: [Outreachy kernel] Re: [PATCH] staging: iio: ade7753: replace
mlock with driver private lock
On Sun, Mar 19, 2017 at 4:01 PM, Jonathan Cameron <jic23@...nel.org> wrote:
> On 17/03/17 09:32, Gargi Sharma wrote:
>> On Mon, Mar 13, 2017 at 5:30 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>>
>>> On 03/12/2017 02:32 PM, simran singhal wrote:
>>>> The IIO subsystem is redefining iio_dev->mlock to be used by
>>>> the IIO core only for protecting device operating mode changes.
>>>> ie. Changes between INDIO_DIRECT_MODE, INDIO_BUFFER_* modes.
>>>>
>>>> In this driver, mlock was being used to protect hardware state
>>>> changes. Replace it with a lock in the devices global data.
>>>>
>>>> Fix some coding style issues related to white space also.
>>>>
>>>> Signed-off-by: simran singhal <singhalsimran0@...il.com>
>>>> ---
>>>> drivers/staging/iio/meter/ade7753.c | 14 ++++++++------
>>>> 1 file changed, 8 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/staging/iio/meter/ade7753.c b/drivers/staging/iio/meter/ade7753.c
>>>> index dfd8b71..ca99d82 100644
>>>> --- a/drivers/staging/iio/meter/ade7753.c
>>>> +++ b/drivers/staging/iio/meter/ade7753.c
>>>> @@ -81,12 +81,14 @@
>>>> * @tx: transmit buffer
>>>> * @rx: receive buffer
>>>> * @buf_lock: mutex to protect tx and rx
>>>> + * @lock: protect sensor state
>>>
>>> It might make sense to reuse the existing lock which currently protects the
>>> read/write functions. You can do this by introducing a variant of
>>> ade7753_spi_{read,write}_reg_16() that does not take a lock and use these to
>>> implement the read-modify-write cycle in a protected section.
>>
>> There are other read/write functions for example,
>> ade7753_spi_{read/write}_reg_8 that use the mutex as well. Should a
>> variant of these functions be introduced as well? Also, how does one
>> go about implementing RMW inside a protected section.
> Hmm. Simran has also been progressing with patches for this.
>
I was trying to work through a patch for ade7754. So ran into the same
problem :)
> You raise a good question. There are other read/modify/write sequences in
> the driver. They don't have the same issue with potentially deadlocking
> against the buf lock as they are all using the spi subsystems provisions
> for small write/read cycles where buffer protection is handled internally.
>
> So let us address the cases in turn:
>
> static int ade7753_reset(struct device *dev)
> {
> u16 val;
> int ret;
>
> ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
> if (ret)
> return ret;
>
> val |= BIT(6); /* Software Chip Reset */
>
> return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
> }
> This is only called in the device initialization. At that point
> we should be fine in assuming no parallel calls. Crucial point
> is it is before the call to iio_device_register which exposes
> the userspace interfaces.
>
> static int ade7753_set_irq(struct device *dev, bool enable)
> {
> int ret;
> u8 irqen;
>
> ret = ade7753_spi_read_reg_8(dev, ADE7753_IRQEN, &irqen);
> if (ret)
> goto error_ret;
>
> if (enable)
> irqen |= BIT(3); /* Enables an interrupt when a data is
> * present in the waveform register
> */
> else
> irqen &= ~BIT(3);
>
> ret = ade7753_spi_write_reg_8(dev, ADE7753_IRQEN, irqen);
>
> error_ret:
> return ret;
> }
>
> This one is actually safe because it is the only function that
> modifies that particular register.
>
> /* Power down the device */
> static int ade7753_stop_device(struct device *dev)
> {
> u16 val;
> int ret;
>
> ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
> if (ret)
> return ret;
>
> val |= BIT(4); /* AD converters can be turned off */
>
> return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
> }
>
> Only called in remove (after userspace interfaces have been
> removed by the iio_device_unregister call so also should not
> be running concurrently with much else.
>
The only nested lock here is ade7754_spi_write_reg_16, so as long as
that is refactored, it'll be fine.
> So I think all the other cases are safe. Perhaps it would have
> been better to have had a lock around them, purely to make
> the code more resilient against future changes though.
> Probably a job to do as part of a larger scale pile of work
> on that driver rather than as a one off patch.
Another question that I have is why are we writing inside a read
function(ade7754_spi_read_reg_24)?
static int ade7754_spi_read_reg_24(struct device *dev,
u8 reg_address, u32 *val)
{
struct iio_dev *indio_dev = dev_to_iio_dev(dev);
struct ade7754_state *st = iio_priv(indio_dev);
int ret;
struct spi_transfer xfers[] = {
{
.tx_buf = st->tx,
.rx_buf = st->rx,
.bits_per_word = 8,
.len = 4,
},
};
mutex_lock(&st->buf_lock);
st->tx[0] = ADE7754_READ_REG(reg_address);
st->tx[1] = 0;
st->tx[2] = 0;
st->tx[3] = 0;
ret = spi_sync_transfer(st->us, xfers, ARRAY_SIZE(xfers));
if (ret) {
dev_err(&st->us->dev, "problem when reading 24 bit
register 0x%02X",
reg_address);
goto error_ret;
}
*val = (st->rx[1] << 16) | (st->rx[2] << 8) | st->rx[3];
error_ret:
mutex_unlock(&st->buf_lock);
return ret;
}
Thanks!
Gargi
>
> Jonathan
>
>
>
>
>
>
>
>>
>>
>>>
>>> Looking through the driver there seem to be other places as well that do
>>> read-modify-write that should be protected by a lock, but currently are not.
>>> This might be a good task.
>>
>> Am I right in understanding that we want to introduce mutex lock for
>> writes in other drivers as well?
>>
>> Thanks,
>> Gargi
>>>
>>>> **/
>>>> struct ade7753_state {
>>>> - struct spi_device *us;
>>>> - struct mutex buf_lock;
>>>> - u8 tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>>> - u8 rx[ADE7753_MAX_RX];
>>>> + struct spi_device *us;
>>>> + struct mutex buf_lock;
>>>> + struct mutex lock; /* protect sensor state */
>>>> + u8 tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>>> + u8 rx[ADE7753_MAX_RX];
>>>> };
>>>>
>>>> static int ade7753_spi_write_reg_8(struct device *dev,
>>>> @@ -484,7 +486,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>>> if (!val)
>>>> return -EINVAL;
>>>>
>>>> - mutex_lock(&indio_dev->mlock);
>>>> + mutex_lock(&st->lock);
>>>>
>>>> t = 27900 / val;
>>>> if (t > 0)
>>>> @@ -505,7 +507,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>>> ret = ade7753_spi_write_reg_16(dev, ADE7753_MODE, reg);
>>>>
>>>> out:
>>>> - mutex_unlock(&indio_dev->mlock);
>>>> + mutex_unlock(&st->lock);
>>>>
>>>> return ret ? ret : len;
>>>> }
>>>>
>>>
>>> --
>> --
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