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HID: uhid: allow poll()'ing on uhid devices

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As long as the internal buffer is not empty, we return POLLIN to
user-space.

uhid->head and uhid->tail are no atomics so the comparison may return
inexact results. However, this doesn't matter here as user-space would
need to poll() in two threads simultaneously to trigger this. And in this
case it doesn't matter if a cached result is returned or the exact new
result as user-space does not know which thread returns first from poll()
and the following read(). So it is safe to compare the values without
locking.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
This commit is contained in:
David Herrmann 2012-06-10 15:16:15 +02:00 committed by Jiri Kosina
parent ace3d8614a
commit 1f9dec1e01
1 changed files with 7 additions and 0 deletions
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7
drivers/hid/uhid.c
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@ -117,6 +117,13 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
static unsigned int uhid_char_poll(struct file *file, poll_table *wait)
{
struct uhid_device *uhid = file->private_data;
poll_wait(file, &uhid->waitq, wait);
if (uhid->head != uhid->tail)
return POLLIN | POLLRDNORM;
return 0;
}