linux-kernel - Re: [PATCH v5 3/5] counter: Add character device interface

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Date:   Sun, 25 Oct 2020 09:18:09 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     David Lechner <david@...hnology.com>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v5 3/5] counter: Add character device interface

On Tue, Oct 20, 2020 at 11:06:42AM -0500, David Lechner wrote:
> On 10/18/20 11:58 AM, William Breathitt Gray wrote:
> > On Wed, Oct 14, 2020 at 05:40:44PM -0500, David Lechner wrote:
> >> On 9/26/20 9:18 PM, William Breathitt Gray wrote:
> >>> +static ssize_t counter_chrdev_read(struct file *filp, char __user *buf,
> >>> +				   size_t len, loff_t *f_ps)
> >>> +{
> >>> +	struct counter_device *const counter = filp->private_data;
> >>> +	int err;
> >>> +	unsigned long flags;
> >>> +	unsigned int copied;
> >>> +
> >>> +	if (len < sizeof(struct counter_event))
> >>> +		return -EINVAL;
> >>> +
> >>> +	do {
> >>> +		if (kfifo_is_empty(&counter->events)) {
> >>> +			if (filp->f_flags & O_NONBLOCK)
> >>> +				return -EAGAIN;
> >>> +
> >>> +			err = wait_event_interruptible(counter->events_wait,
> >>> +					!kfifo_is_empty(&counter->events));
> >>> +			if (err)
> >>> +				return err;
> >>> +		}
> >>> +
> >>> +		raw_spin_lock_irqsave(&counter->events_lock, flags);
> >>> +		err = kfifo_to_user(&counter->events, buf, len, &copied);
> >>> +		raw_spin_unlock_irqrestore(&counter->events_lock, flags);
> >>> +		if (err)
> >>> +			return err;
> >>> +	} while (!copied);
> >>> +
> >>> +	return copied;
> >>> +}
> >>
> >> All other uses of kfifo_to_user() I saw use a mutex instead of spin
> >> lock. I don't see a reason for disabling interrupts here.
> > 
> > The Counter character device interface is special in this case because
> > counter->events could be accessed from an interrupt context. This is
> > possible if counter_push_event() is called for an interrupt (as is the
> > case for the 104_quad_8 driver). In this case, we can't use mutex
> > because we can't sleep in an interrupt context, so our only option is to
> > use spin lock.
> > 
> 
> 
> The way I understand it, locking is only needed for concurrent readers
> and locking between reader and writer is not needed.

You're right, it does say in the kfifo.h comments that with only one
concurrent reader and one current write, we don't need extra locking to
use these macros. Because we only have one kfifo_to_user() operating on
counter->events, does that mean we don't need locking at all here for
the counter_chrdev_read() function?

William Breathitt Gray

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