>
>
> On Fri, 19 Nov 1999, Benjamin C.R. LaHaise wrote:
> >
> > Sure. Does the following sound?
> >
> > void down_read(struct rw_semaphore *sem);
> > void down_write(struct rw_semaphore *sem);
> > void up_read(struct rw_semaphore *sem);
> > void up_write(struct rw_semaphore *sem);
>
> Looks fine.
>
> > Looking at existing semaphores and rwlocks, it looks like a quick and
> > painless beast to create.
>
> Yeah, sure.
>
> Especially considering that the current semaphore code has been basically
> re-written three times to get it stable ;)
>
> But yes, mixing judicious amounts of the current semaphore code together
> with the rw-spinlock code should result in something working.
>
> "Just add brains".
IIRC, there was an implementation and analysis in Vahalia. Streamlining it
shouldn't be too hard (I have something like that in AFFS patches, but
there I didn't need {up,down}_write and did need upgrade/downgrade. IOW,
writrs always started as readers). IIRC, Vahalia uses two queues, counters
a-la rwlock_t has and counters of pending readers and writers. down_read()
blocks if there are pending writers and up_write() either wakes one writer
or (if ->writers==0) all pending readers.
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