From: William Breathitt Gray <vilhelm.gray@gmail.com>
To: David Lechner <david@lechnology.com>
Cc: jic23@kernel.org, kamel.bouhara@bootlin.com,
gwendal@chromium.org, alexandre.belloni@bootlin.com,
linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-stm32@st-md-mailman.stormreply.com,
linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com,
patrick.havelange@essensium.com, fabrice.gasnier@st.com,
mcoquelin.stm32@gmail.com, alexandre.torgue@st.com
Subject: Re: [PATCH v5 3/5] counter: Add character device interface
Date: Sun, 25 Oct 2020 13:53:22 -0400 [thread overview]
Message-ID: <20201025175322.GA14219@shinobu> (raw)
In-Reply-To: <e0b7989f-6a99-0fae-471c-8d06c8e951b0@lechnology.com>
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On Sun, Oct 25, 2020 at 11:34:43AM -0500, David Lechner wrote:
> On 10/25/20 8:18 AM, William Breathitt Gray wrote:
> > 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
> >
>
> Even if we have the policy that only one file handle to the chrdev
> can be open at a time, it is still possible that the it could be
> read from multiple threads. So it I think it makes sense to keep
> it just to be safe.
All right, I'll keep the locks in the code for now to keep it safe in
case we have multiple threads reading.
William Breathitt Gray
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next prev parent reply other threads:[~2020-10-25 17:53 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-27 2:18 [PATCH v5 0/5] Introduce the Counter character device interface William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 2/5] docs: counter: Update to reflect sysfs internalization William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 3/5] counter: Add character device interface William Breathitt Gray
2020-10-14 1:40 ` David Lechner
2020-10-18 16:49 ` William Breathitt Gray
2020-10-20 15:53 ` David Lechner
2020-10-25 12:55 ` William Breathitt Gray
2020-10-25 16:36 ` David Lechner
2020-10-14 17:43 ` David Lechner
2020-10-14 19:05 ` William Breathitt Gray
2020-10-14 22:32 ` David Lechner
2020-10-14 22:40 ` David Lechner
2020-10-18 16:58 ` William Breathitt Gray
2020-10-20 16:06 ` David Lechner
2020-10-25 13:18 ` William Breathitt Gray
2020-10-25 16:34 ` David Lechner
2020-10-25 17:53 ` William Breathitt Gray [this message]
2020-09-27 2:18 ` [PATCH v5 4/5] docs: counter: Document " William Breathitt Gray
2020-10-08 8:09 ` Pavel Machek
2020-10-08 12:28 ` William Breathitt Gray
2020-10-12 17:04 ` David Lechner
2020-10-13 18:58 ` William Breathitt Gray
2020-10-13 19:08 ` David Lechner
2020-10-13 19:27 ` William Breathitt Gray
2020-09-27 2:18 ` [PATCH v5 5/5] counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8 William Breathitt Gray
2020-10-14 0:13 ` David Lechner
2020-10-18 14:50 ` William Breathitt Gray
2020-10-13 0:35 ` [PATCH v5 0/5] Introduce the Counter character device interface David Lechner
2020-10-18 14:14 ` William Breathitt Gray
[not found] ` <e38f6dc3a08bf2510034334262776a6ed1df8b89.1601170670.git.vilhelm.gray@gmail.com>
2020-10-13 2:15 ` [PATCH v5 1/5] counter: Internalize sysfs interface code David Lechner
2020-10-18 14:49 ` William Breathitt Gray
2020-10-20 15:38 ` David Lechner
2020-10-23 13:12 ` William Breathitt Gray
2020-10-15 1:38 ` David Lechner
2020-10-18 17:00 ` William Breathitt Gray
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