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mutex: Do not unnecessarily deal with waiters
Upon entering the slowpath, we immediately attempt to acquire the lock by checking if it is already unlocked. If we are lucky enough that this is the case, then we don't need to deal with any waiter related logic. Furthermore any checks for an empty wait_list are unnecessary as we already know that count is non-negative and hence no one is waiting for the lock. Move the count check and xchg calls to be done before any waiters are setup - including waiter debugging. Upon failure to acquire the lock, the xchg sets the counter to 0, instead of -1 as it was originally. This can be done here since we set it back to -1 right at the beginning of the loop so other waiters are woken up when the lock is released. When tested on a 8-socket (80 core) system against a vanilla 3.10-rc1 kernel, this patch provides some small performance benefits (+2-6%). While it could be considered in the noise level, the average percentages were stable across multiple runs and no performance regressions were seen. Two big winners, for small amounts of users (10-100), were the short and compute workloads had a +19.36% and +%15.76% in jobs per minute. Also change some break statements to 'goto slowpath', which IMO makes a little more intuitive to read. Signed-off-by:Davidlohr Bueso <davidlohr.bueso@hp.com> Acked-by: