[RFC] watchdog ->release() races

[Al Viro <viro@ZenIV.linux.org.uk>]

	Watchdog drivers tend to do something like that:

foo_open()
{
	if (test_and_set_bit(0, &foo_is_open))
		return -EBUSY;
	...
}

foo_write()
{
	...
	assign foo_expect_close
	...
}

foo_release()
{
	look at foo_expect_close, act accordingly
	clear_bit(0, &foo_is_open);
	foo_expect_close = 0;
}

OK, so it tries to make sure that there's only one opened struct file for
the device; fair enough, but what happens if we have
task A: open()/write()/close()
task B: open()/write()/close()
with task A losing CPU just between clear_bit() and clearing foo_expect_close?
If it regains CPU just after write() done by task B, we'll get foo_expect_close
unexpectedly cleared.

It's obviously racy; I'm not sure if we care about that race, but if we
do, there's about 80 drivers that need to be fixed...

Comments?