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Received: from [2600:1700:4830:165f::19e] (port=37582) by vern.gendns.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kWiyu-00056T-HR; Sun, 25 Oct 2020 12:34:56 -0400 Subject: Re: [PATCH v5 3/5] counter: Add character device interface To: William Breathitt Gray References: <00be1fccc672c5207f3b04fe4cc09c29e22641f4.1601170670.git.vilhelm.gray@gmail.com> <20201018165822.GE231549@shinobu> <20201025131809.GB3458@shinobu> From: David Lechner Message-ID: Date: Sun, 25 Oct 2020 11:34:43 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20201025131809.GB3458@shinobu> Content-Language: en-US X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - vern.gendns.com X-AntiAbuse: Original Domain - lists.infradead.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - lechnology.com X-Get-Message-Sender-Via: vern.gendns.com: authenticated_id: davidmain+lechnology.com/only user confirmed/virtual account not confirmed X-Authenticated-Sender: vern.gendns.com: davidmain@lechnology.com X-Source: X-Source-Args: X-Source-Dir: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201025_123542_289349_948BBA09 X-CRM114-Status: GOOD ( 18.87 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.torgue@st.com, linux-iio@vger.kernel.org, patrick.havelange@essensium.com, alexandre.belloni@bootlin.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mcoquelin.stm32@gmail.com, syednwaris@gmail.com, linux-stm32@st-md-mailman.stormreply.com, jic23@kernel.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel