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

[Waiman Long]

On 03/28/2019 04:47 PM, Linus Torvalds 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.
>
> I think this at a minimum needs to be made very clear in the patch
> that actually lowered the reader count bits, but preferably this just
> needs to be done *before* that patch, so that you don't have a point
> in history where rwlocks are simply not reliable.
>
>                Linus

Thanks for the suggestion. I will move this patch up in the next version.

Cheers,
Longman