Here's a not-so-theoretical question.
I've got a module which registers a struct device. (It represents a
virtual device, not a real one, but that doesn't matter.) Obviously the
module's exit routine has to wait until the release() routine for that
device has been invoked -- if it returned too early then the release()
call would oops.
How should it wait?
The most straightforward approach is to use a struct completion, like
this:
static struct {
struct device dev;
...
} my_dev;
static DECLARE_COMPLETION(my_completion);
static void my_release(struct device *dev)
{
complete(&my_completion);
}
static void __exit my_exit(void)
{
device_unregister(&my_dev.dev);
wait_for_completion(&my_completion);
}
The problem is that there is no guarantee a context switch won't take
place after my_release() has called complete() and before my_release()
returns. If that happens and my_exit() finishes running, then the module
will be unloaded and the next context switch back to finish off
my_release() will oops.
Other approaches have similar defects. So how can this problem be solved?
Alan Stern
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