Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

[Waiman Long <waiman.long@hp.com>]

On 12/17/2013 02:27 PM, Linus Torvalds wrote:
> On Tue, Dec 17, 2013 at 11:21 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com>  wrote:
>> Looks like xadd() is x86-specific, but this is common code.  One
>> approach would be to do xadd() for other arches, another approach
>> would be to make .rw be an atomic_t rather than a u32.  Making it
>> be atomic_t is probably easiest.  (The cmpxchg()s would then need
>> to be atomic_cmpxchg().)
> Note that "xadd()" has different semantics from "atomic_add_return()".
>
> xadd() returns the original value, while atomic_add_return() returns
> the result of the addition.
>
> In this case, we seem to want the xadd() semantics. I guess we can use
> "atomic_add_return(val,&atomic)-val" and just assume that the compiler
> gets it right (with the addition and the subtraction cancelling out).
> Or maybe we should have a "atomic_add_return_original()" with xadd
> semantics?
>
>                 Linus

I will use the atomic_add_return() for the combined count. Actually what 
the code is looking for in the returned value is the state of the writer 
byte. So it doesn't really matter if _QR_BIAS has or hasn't been added 
to the count.

-Longman