Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

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

On 02/18/2014 04:34 PM, Peter Zijlstra wrote:
> On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
>> The #ifdef is harder to take away here. The point is that doing a 32-bit
>> exchange may accidentally steal the lock with the additional code to handle
>> that. Doing a 16-bit exchange, on the other hand, will never steal the lock
>> and so don't need the extra handling code. I could construct a function with
>> different return values to handle the different cases if you think it will
>> make the code easier to read.
> Does it really pay to use xchg() with all those fixup cases? Why not
> have a single cmpxchg() loop that does just the exact atomic op you
> want?

The main reason for using xchg instead of cmpxchg is its performance 
impact when the lock is heavily contended. Under those circumstances, a 
task may need to do several tries of read+atomic-RMV before getting it 
right. This may cause a lot of cacheline contention. With xchg, we need 
at most 2 atomic ops. Using cmpxchg() does simplify the code a bit at 
the expense of performance with heavy contention.

-Longman