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From: Kent Gibson <warthog618@gmail.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH
Date: Thu, 25 Jun 2020 06:58:03 +0800	[thread overview]
Message-ID: <20200624225803.GA3600@sol> (raw)
In-Reply-To: <20200624155714.GB8622@sol>

On Wed, Jun 24, 2020 at 11:57:14PM +0800, Kent Gibson wrote:
> On Wed, Jun 24, 2020 at 05:46:33PM +0300, Andy Shevchenko wrote:
> > On Tue, Jun 23, 2020 at 7:03 AM Kent Gibson <warthog618@gmail.com> wrote:
> > >
> > > Merge separate usage of test_bit/set_bit into test_and_set_bit to remove
> > > the possibility of a race between the test and set.
> > >
> > > Similarly test_bit and clear_bit.
> > >
> > > In the existing code it is possible for two threads to race past the
> > > test_bit and then set or clear the watch bit, and neither return EBUSY.
> > 
> > I stumbled over this myself, but...
> > 
> > > -               if (test_bit(hwgpio, gcdev->watched_lines))
> > > +               if (test_and_set_bit(hwgpio, gcdev->watched_lines))
> > >                         return -EBUSY;
> > >
> > >                 gpio_desc_to_lineinfo(desc, &lineinfo);
> > > @@ -897,7 +897,6 @@ static long gpio_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > >                 if (copy_to_user(ip, &lineinfo, sizeof(lineinfo)))
> > >                         return -EFAULT;
> > >
> > > -               set_bit(hwgpio, gcdev->watched_lines);
> > >                 return 0;
> > 
> > ...I think it's not an equivalent despite races involved. If you set
> > bit and return error code, you will have the wrong state.
> > 
> 
> Not quite sure what you mean.  There is only an error if the bit is
> already set, so you've changed nothing.
> 
> And the watched state is not part of the lineinfo, so the state returned is
> the same either way.
> 

Perhaps you are referring to the case where the copy_to_user fails?
To be honest I considered that to be so unlikely that I ignored it.
Is there a relevant failure mode that I'm missing?

Cheers,
Kent.

  reply	other threads:[~2020-06-24 22:58 UTC|newest]

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-23  4:00 [PATCH 00/22] gpio: cdev: add uAPI V2 Kent Gibson
2020-06-23  4:00 ` [PATCH 01/22] gpiolib: move gpiolib-sysfs function declarations into their own header Kent Gibson
2020-06-24 13:34   ` Bartosz Golaszewski
2020-06-24 13:38     ` Kent Gibson
2020-06-23  4:00 ` [PATCH 02/22] gpiolib: cdev: sort includes Kent Gibson
2020-06-24 13:34   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 03/22] gpiolib: cdev: minor indentation fixes Kent Gibson
2020-06-24 13:35   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 04/22] gpiolib: cdev: refactor gpiohandle_flags_to_desc_flags Kent Gibson
2020-06-24 13:51   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 05/22] gpiolib: cdev: rename 'filep' and 'filp' to 'file' to be consistent with other use Kent Gibson
2020-06-24 13:52   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 06/22] gpiolib: cdev: rename numdescs to num_descs Kent Gibson
2020-06-24 13:53   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 07/22] gpiolib: cdev: remove pointless decrement of i Kent Gibson
2020-06-24 13:53   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 08/22] gpiolib: cdev: complete the irq/thread timestamp handshake Kent Gibson
2020-06-24 14:00   ` Bartosz Golaszewski
2020-06-24 14:08     ` Kent Gibson
2020-06-25  9:44       ` Bartosz Golaszewski
2020-06-25 10:01         ` Kent Gibson
2020-06-30  9:08           ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 09/22] gpiolib: cdev: rename priv to gcdev Kent Gibson
2020-06-24 14:04   ` Bartosz Golaszewski
2020-06-24 14:19     ` Kent Gibson
2020-06-24 14:20       ` Bartosz Golaszewski
2020-06-24 23:16         ` Kent Gibson
2020-06-25 10:26           ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH Kent Gibson
2020-06-24 14:05   ` Bartosz Golaszewski
2020-06-24 14:46   ` Andy Shevchenko
2020-06-24 15:57     ` Kent Gibson
2020-06-24 22:58       ` Kent Gibson [this message]
2020-06-25  8:44         ` Andy Shevchenko
2020-06-25  9:13           ` Kent Gibson
2020-06-25  9:23             ` Andy Shevchenko
2020-06-25  9:36               ` Kent Gibson
2020-06-23  4:00 ` [PATCH 11/22] gpiolib: cdev: remove recalculation of offset Kent Gibson
2020-06-30  8:56   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 12/22] gpio: uapi: define GPIO_MAX_NAME_SIZE for array sizes Kent Gibson
2020-06-24 14:13   ` Bartosz Golaszewski
2020-06-23  4:00 ` [PATCH 13/22] gpio: uapi: define uAPI V2 Kent Gibson
2020-06-24 14:33   ` Andy Shevchenko
2020-06-24 15:40     ` Kent Gibson
2020-06-26 14:02       ` Kent Gibson
2020-06-24 14:36   ` Bartosz Golaszewski
2020-06-24 23:58     ` Kent Gibson
2020-06-23  4:00 ` [PATCH 14/22] gpiolib: make cdev a build option Kent Gibson
2020-06-29 14:25   ` Bartosz Golaszewski
2020-06-23  4:01 ` [PATCH 15/22] gpiolib: add build option for CDEV V1 ABI Kent Gibson
2020-06-29 14:26   ` Bartosz Golaszewski
2020-06-23  4:01 ` [PATCH 16/22] gpiolib: cdev: add V2 uAPI implementation to parity with V1 Kent Gibson
2020-06-23 17:44   ` Dan Carpenter
2020-06-23 23:23     ` Kent Gibson
2020-06-23  4:01 ` [PATCH 17/22] gpiolib: cdev: report edge detection in lineinfo Kent Gibson
2020-06-23  4:01 ` [PATCH 18/22] gpiolib: cdev: support setting debounce Kent Gibson
2020-06-23  4:01 ` [PATCH 19/22] gpio: uapi: document uAPI V1 as deprecated Kent Gibson
2020-06-23  4:01 ` [PATCH 20/22] tools: gpio: switch tools to V2 uAPI Kent Gibson
2020-06-23  4:01 ` [PATCH 21/22] tools: gpio: add debounce support to gpio-event-mon Kent Gibson
2020-06-23  4:01 ` [PATCH 22/22] tools: gpio: support monitoring multiple lines Kent Gibson
2020-06-30 10:43   ` Bartosz Golaszewski

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