From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 71478C00523 for ; Wed, 8 Jan 2020 11:06:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4433820673 for ; Wed, 8 Jan 2020 11:06:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727088AbgAHLGl (ORCPT ); Wed, 8 Jan 2020 06:06:41 -0500 Received: from mga11.intel.com ([192.55.52.93]:42333 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726276AbgAHLGk (ORCPT ); Wed, 8 Jan 2020 06:06:40 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 Jan 2020 03:06:40 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,409,1571727600"; d="scan'208";a="421415431" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by fmsmga005.fm.intel.com with ESMTP; 08 Jan 2020 03:06:38 -0800 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1ip9Ad-0001gj-Dp; Wed, 08 Jan 2020 13:06:39 +0200 Date: Wed, 8 Jan 2020 13:06:39 +0200 From: Andy Shevchenko To: Bartosz Golaszewski Cc: Kent Gibson , Linus Walleij , Greg Kroah-Hartman , linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org, Bartosz Golaszewski Subject: Re: [PATCH v4 09/13] gpiolib: rework the locking mechanism for lineevent kfifo Message-ID: <20200108110639.GL32742@smile.fi.intel.com> References: <20191224120709.18247-1-brgl@bgdev.pl> <20191224120709.18247-10-brgl@bgdev.pl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191224120709.18247-10-brgl@bgdev.pl> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-gpio-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-gpio@vger.kernel.org On Tue, Dec 24, 2019 at 01:07:05PM +0100, Bartosz Golaszewski wrote: > From: Bartosz Golaszewski > > The read_lock mutex is supposed to prevent collisions between reading > and writing to the line event kfifo but it's actually only taken when > the events are being read from it. > > Drop the mutex entirely and reuse the spinlock made available to us in > the waitqueue struct. Take the lock whenever the fifo is modified or > inspected. Drop the call to kfifo_to_user() and instead first extract > the new element from kfifo when the lock is taken and only then pass > it on to the user after the spinlock is released. Sounds like a plausible approach. Reviewed-by: Andy Shevchenko > > Signed-off-by: Bartosz Golaszewski > --- > drivers/gpio/gpiolib.c | 64 +++++++++++++++++++++++------------------- > 1 file changed, 35 insertions(+), 29 deletions(-) > > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c > index 81d5eda4de7d..a859c0813e0d 100644 > --- a/drivers/gpio/gpiolib.c > +++ b/drivers/gpio/gpiolib.c > @@ -788,8 +788,6 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip) > * @irq: the interrupt that trigger in response to events on this GPIO > * @wait: wait queue that handles blocking reads of events > * @events: KFIFO for the GPIO events > - * @read_lock: mutex lock to protect reads from colliding with adding > - * new events to the FIFO > * @timestamp: cache for the timestamp storing it between hardirq > * and IRQ thread, used to bring the timestamp close to the actual > * event > @@ -802,7 +800,6 @@ struct lineevent_state { > int irq; > wait_queue_head_t wait; > DECLARE_KFIFO(events, struct gpioevent_data, 16); > - struct mutex read_lock; > u64 timestamp; > }; > > @@ -818,7 +815,7 @@ static __poll_t lineevent_poll(struct file *filep, > > poll_wait(filep, &le->wait, wait); > > - if (!kfifo_is_empty(&le->events)) > + if (!kfifo_is_empty_spinlocked_noirqsave(&le->events, &le->wait.lock)) > events = EPOLLIN | EPOLLRDNORM; > > return events; > @@ -831,43 +828,52 @@ static ssize_t lineevent_read(struct file *filep, > loff_t *f_ps) > { > struct lineevent_state *le = filep->private_data; > - unsigned int copied; > + struct gpioevent_data event; > + ssize_t bytes_read = 0; > int ret; > > - if (count < sizeof(struct gpioevent_data)) > + if (count < sizeof(event)) > return -EINVAL; > > do { > + spin_lock(&le->wait.lock); > if (kfifo_is_empty(&le->events)) { > - if (filep->f_flags & O_NONBLOCK) > + if (bytes_read) { > + spin_unlock(&le->wait.lock); > + return bytes_read; > + } > + > + if (filep->f_flags & O_NONBLOCK) { > + spin_unlock(&le->wait.lock); > return -EAGAIN; > + } > > - ret = wait_event_interruptible(le->wait, > + ret = wait_event_interruptible_locked(le->wait, > !kfifo_is_empty(&le->events)); > - if (ret) > + if (ret) { > + spin_unlock(&le->wait.lock); > return ret; > + } > } > > - if (mutex_lock_interruptible(&le->read_lock)) > - return -ERESTARTSYS; > - ret = kfifo_to_user(&le->events, buf, count, &copied); > - mutex_unlock(&le->read_lock); > - > - if (ret) > - return ret; > - > - /* > - * If we couldn't read anything from the fifo (a different > - * thread might have been faster) we either return -EAGAIN if > - * the file descriptor is non-blocking, otherwise we go back to > - * sleep and wait for more data to arrive. > - */ > - if (copied == 0 && (filep->f_flags & O_NONBLOCK)) > - return -EAGAIN; > + ret = kfifo_out(&le->events, &event, 1); > + spin_unlock(&le->wait.lock); > + if (ret != 1) { > + /* > + * This should never happen - we were holding the lock > + * from the moment we learned the fifo is no longer > + * empty until now. > + */ > + ret = -EIO; > + break; > + } > > - } while (copied == 0); > + if (copy_to_user(buf + bytes_read, &event, sizeof(event))) > + return -EFAULT; > + bytes_read += sizeof(event); > + } while (count >= bytes_read + sizeof(event)); > > - return copied; > + return bytes_read; > } > > static int lineevent_release(struct inode *inode, struct file *filep) > @@ -969,7 +975,8 @@ static irqreturn_t lineevent_irq_thread(int irq, void *p) > return IRQ_NONE; > } > > - ret = kfifo_put(&le->events, ge); > + ret = kfifo_in_spinlocked_noirqsave(&le->events, &ge, > + 1, &le->wait.lock); > if (ret) > wake_up_poll(&le->wait, EPOLLIN); > > @@ -1084,7 +1091,6 @@ static int lineevent_create(struct gpio_device *gdev, void __user *ip) > > INIT_KFIFO(le->events); > init_waitqueue_head(&le->wait); > - mutex_init(&le->read_lock); > > /* Request a thread to read the events */ > ret = request_threaded_irq(le->irq, > -- > 2.23.0 > -- With Best Regards, Andy Shevchenko