On Thu, Feb 13, 2003 at 09:09:18PM +0100, Adam ENDRODI wrote:
>
> 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()?
No. Readlock can't be upgraded to a write one and the reason for this is
principial. Consider two concurent readers. Both hold the read lock and
then both decide to upgrade it to a write lock. But one can only be
granted a write lock if noone is holding a read lock. So you have
a deadlock.
-------------------------------------------------------------------------------
Jan 'Bulb' Hudec <bulb@ucw.cz>
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