linux-kernel - Re: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO

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Date:   Wed, 24 Jun 2020 17:46:33 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Kent Gibson <warthog618@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH

On Tue, Jun 23, 2020 at 7:03 AM Kent Gibson <warthog618@...il.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.

...

> -               if (!test_bit(hwgpio, gcdev->watched_lines))
> +               if (!test_and_clear_bit(hwgpio, gcdev->watched_lines))
>                         return -EBUSY;
>
> -               clear_bit(hwgpio, gcdev->watched_lines);
>                 return 0;

OTOH, this is okay as long as we have no code in between. So, I really
prefer something better to do such checks.
(Alas, I can't come up with a proposal right now)

-- 
With Best Regards,
Andy Shevchenko