[PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64

[Peter Zijlstra]

On Thu, Feb 07, 2019 at 02:07:19PM -0500, Waiman Long wrote:
> On 32-bit architectures, there aren't enough bits to hold both.
> 64-bit architectures, however, can have enough bits to do that. For
> x86-64, the physical address can use up to 52 bits. That is 4PB of
> memory. That leaves 12 bits available for other use. The task structure
> pointer is also aligned to the L1 cache size. That means another 6 bits
> (64 bytes cacheline) will be available. Reserving 2 bits for status
> flags, we will have 16 bits for the reader count.  That can supports
> up to (64k-1) readers.

*groan*...

So take qrwlock's idea for a queue, then make the count value (similar
to the new mutex); that is have a bit0 be a r/w bit, when w bits 6-N are
owner, when r they are reader-count. bit1 can be a pending bit, bit2 a
handoff bit etc..

That should fit and work on 32bit and 64bit without issue.

I have a half-arsed rwsem-atomic.c somewhere that does just that. I just
never got around to doing all the optimistic spin and steal crap that
makes our current rwsem fly.

And that nicely gets rid of that mind bending BIAS crud.