On Tue, Feb 18, 2014 at 07:50:13PM -0500, Waiman Long wrote: > 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. Have you actually measured this? -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
