On Fri, Mar 30, 2012 at 11:06:19PM -0700, Dmitry Torokhov wrote:
> From: David Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
>
> Consider two threads calling read() on the same uinput-fd, both
> non-blocking. Assume there is data-available so both will simultaneously
> pass:
> udev->head == udev->tail
>
> Then the first thread goes to sleep and the second one pops the message
> from the queue. Now assume udev->head == udev->tail. If the first thread
> wakes up it will call wait_event_*() and sleep in the waitq. This
> effectively turns the non-blocking FD into a blocking one.
>
> We fix this by never calling wait_event_*() for non-blocking FDs hence we
> will never sleep in the waitq here.
>
> Also, if we fail to retrieve an event because it was "stolen" by another
> thread, we will return -EAGAIN instead of 0 in case of nonblocking read.
>
> Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxxxxxxx>
> Signed-off-by: Dmitry Torokhov <dtor@xxxxxxx>
> ---
> drivers/input/misc/uinput.c | 26 ++++++++++++++------------
> 1 files changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
> index eb9723a..5339c1d 100644
> --- a/drivers/input/misc/uinput.c
> +++ b/drivers/input/misc/uinput.c
> @@ -460,16 +460,13 @@ static ssize_t uinput_read(struct file *file, char __user *buffer, size_t count,
> if (count < input_event_size())
> return -EINVAL;
>
> - if (udev->state != UIST_CREATED)
> - return -ENODEV;
> -
> - if (udev->head == udev->tail && (file->f_flags & O_NONBLOCK))
> - return -EAGAIN;
> -
> - retval = wait_event_interruptible(udev->waitq,
> - udev->head != udev->tail || udev->state != UIST_CREATED);
> - if (retval)
> - return retval;
> + if (!(file->f_flags & O_NONBLOCK)) {
> + retval = wait_event_interruptible(udev->waitq,
> + udev->head != udev->tail ||
> + udev->state != UIST_CREATED);
> + if (retval)
> + return retval;
> + }
no. if the state is not UIST_CREATED, it should return ENODEV, not EAGAIN.
and you're not checking it if O_NONBLOCK is present.
--
Aristeu
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