linux-kernel - Re: [PATCH] Input: serio_raw - ensure we don't block in non-blocking read

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Date:	Tue, 17 Apr 2012 16:56:11 +0800
From:	Che-liang Chiou <clchiou@...omium.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>,
	David Herrmann <dh.herrmann@...glemail.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: serio_raw - ensure we don't block in non-blocking read

I didn't test the patch, but the logic looks fine to me.

On Wed, Apr 4, 2012 at 4:24 AM, Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
> Avoid calling wait_event_interruptible() if client requested non-blocking
> read, since it is not guaranteed that another thread will not consume
> event after we checked if serio_raw->head != serio_raw->tail.
>
> Also ensure we do not return 0 but keep waiting instead in blocking case,
> when another thread steals "our" byte.
>
> Signed-off-by: Dmitry Torokhov <dtor@...l.ru>
> ---
>  drivers/input/serio/serio_raw.c |   43 ++++++++++++++++++++++----------------
>  1 files changed, 25 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/input/serio/serio_raw.c b/drivers/input/serio/serio_raw.c
> index 4494233..094d6fb 100644
> --- a/drivers/input/serio/serio_raw.c
> +++ b/drivers/input/serio/serio_raw.c
> @@ -165,31 +165,38 @@ static ssize_t serio_raw_read(struct file *file, char __user *buffer,
>        struct serio_raw *serio_raw = client->serio_raw;
>        char uninitialized_var(c);
>        ssize_t read = 0;
> -       int retval;
> +       int error = 0;
>
> -       if (serio_raw->dead)
> -               return -ENODEV;
> +       do {
> +               if (serio_raw->dead)
> +                       return -ENODEV;
>
> -       if (serio_raw->head == serio_raw->tail && (file->f_flags & O_NONBLOCK))
> -               return -EAGAIN;
> +               if (serio_raw->head == serio_raw->tail &&
> +                   (file->f_flags & O_NONBLOCK))
> +                       return -EAGAIN;
>
> -       retval = wait_event_interruptible(serio_raw->wait,
> -                       serio_raw->head != serio_raw->tail || serio_raw->dead);
> -       if (retval)
> -               return retval;
> +               if (count == 0)
> +                       break;
>
> -       if (serio_raw->dead)
> -               return -ENODEV;
> +               while (read < count && serio_raw_fetch_byte(serio_raw, &c)) {
> +                       if (put_user(c, buffer++)) {
> +                               error = -EFAULT;
> +                               goto out;
> +                       }
> +                       read++;
> +               }
>
> -       while (read < count && serio_raw_fetch_byte(serio_raw, &c)) {
> -               if (put_user(c, buffer++)) {
> -                       retval = -EFAULT;
> +               if (read)
>                        break;
> -               }
> -               read++;
> -       }
>
> -       return read ?: retval;
> +               if (!(file->f_flags & O_NONBLOCK))
> +                       error = wait_event_interruptible(serio_raw->wait,
> +                                       serio_raw->head != serio_raw->tail ||
> +                                       serio_raw->dead);
> +       } while (!error);
> +
> +out:
> +       return read ?: error;
>  }
>
>  static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
> --
> 1.7.7.6
>
>
> --
> Dmitry