read-only -> read-write locks

[Adam ENDRODI <borso@vekoll.saturnus.vein.hu>]

Hi list,


I came across a situation when I needed to hold a read-only lock
(in fact, a semaphore) but I haven't found a way to upgrade it
to act as a write lock.  I mean, something like this:

struct rwsemaphore sem;

void foo(struct bar *data)
{
	down(&sem);
	if (!data->bar) {
		down_rw(&sem);
		data->bar = kmalloc();
	}
	up(&sem);
}

Is it possible to achieve via rwsemaphores?  If not, is there a
way to emulate the same effect by using atomic_* and semaphore
operations, other than releasing ``sem'' and reaquiring it by
down_write()?

Thanks in advance,
adam

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