Messages in this thread | | | Date | Tue, 18 Feb 2014 19:50:13 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation |
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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
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