On Thu, Mar 28, 2019 at 11:12 AM Waiman Long <[email protected]> wrote:
>
> Reserving 2 bits for status flags,
> we will have 16 bits for the reader count. That can supports up to
> (64k-1) readers.
Explain why that's enough, please.
I could *easily* see more than 64k threads all on the same rwsem, all
at the same time.
Just do a really slow filesystem (think fuse), map a file with lots of
pages, and then fault in one page per thread. Boom. rwsem with more
than 64k concurrent readers.
So I think this approach is completely wrong, and/or needs a *lot* of
explanation why it works.
A small reader count works for the spinning rwlocks because we're
limited to the number of CPU's in the system. For a rwsem? No.
Linus
