Re: [PATCH 12/12] locking/rwsem: Make MSbit of count as guard bit to fail readlock

[Linus Torvalds]

On Thu, Mar 28, 2019 at 1:47 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 28, 2019 at 11:12 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> >
> > With the merging of owner into count for x86-64, there is only 16 bits
> > left for reader count. It is theoretically possible for an application to
> > cause more than 64k readers to acquire a rwsem leading to count overflow.
>
> Ahh, and here's the thing that makes 16 bits work for readers.

Hmm. Does it?

Isn't there a race here? We're adding the READ bias, and then noticing
that it his the guard bit, and then the down_read_failed will make it
all good again.

But this isn't actually done with preemption disabled, so things
*could* get preempted in between, and if we have a huge run of bad
luck, it can still overflow.

Ok, so you need to have a 32k series run of bad luck (and hit
*exactly* the right small preemption point window every time), and I'm
certainly willing to say "yeah, not an issue", but maybe it's still
worth at least documenting?

                  Linus