x86/ticketlock: Fix spin_unlock_wait() livelock
arch_spin_unlock_wait() looks very suboptimal, to the point I think this is just wrong and can lead to livelock: if the lock is heavily contended we can never see head == tail. But we do not need to wait for arch_spin_is_locked() == F. If it is locked we only need to wait until the current owner drops this lock. So we could simply spin until old_head != lock->tickets.head in this case, but .head can overflow and thus we can't check "unlocked" only once before the main loop. Also, the "unlocked" check can ignore TICKET_SLOWPATH_FLAG bit. Signed-off-by: NOleg Nesterov <oleg@redhat.com> Acked-by: NLinus Torvalds <torvalds@linux-foundation.org> Cc: Jeremy Fitzhardinge <jeremy@goop.org> Cc: Paul E.McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Waiman Long <Waiman.Long@hp.com> Link: http://lkml.kernel.org/r/20141201213417.GA5842@redhat.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
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