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

[Waiman Long]

On 02/07/2019 02:45 PM, Peter Zijlstra wrote:
> 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.

Well, the reason for this compromise is to keep using xadd for readers.
Your scheme will certainly work, but we have to use cmpxchg for readers
too. That will have a performance impact especially with multiple
readers contending which I am trying to avoid.

Cheers,
Longman