On Wed, 25 Jan 2012, Mike Galbraith wrote:

On Tue, 2012-01-24 at 10:04 -0600, Sankara Muthukrishnan wrote:
 		spin_unlock_irq(&ctx->wqh.lock);
+#ifndef CONFIG_PREEMPT_RT_BASE

Bah.

 		cpu_relax();
+#else
+		/*
+		 * Current may be an RT task with priority high enough
+		 * to prevent the thread currently _wanting_ to execute
+		 * the timer callback function from receiving the CPU.
+		 */
+		usleep_range(1, 10);

Even more bah.

+#endif
 	}

Index: linux-3.2/fs/timerfd.c
===================================================================

--- linux-3.2.orig/fs/timerfd.c
+++ linux-3.2/fs/timerfd.c

@@ -313,7 +313,7 @@ SYSCALL_DEFINE4(timerfd_settime, int, uf
 		if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
 			break;
 		spin_unlock_irq(&ctx->wqh.lock);
-		cpu_relax();
+		hrtimer_wait_for_timer(&ctx->tmr);
 	}
 
 	/*