Re: [ANNOUNCE] 2.6.15-rc5-hrt2 - hrtimers based high resolution patches

[Steven Rostedt <rostedt@goodmis.org>]

Thomas and Ingo,

I found a bug in 2.6.15-rc5-rt1 that was due to a race in the hrtimers
code.  This bug is most likely in the vanilla hrtimers code as well.

I added a HRTIMER_RUNNING state because there's a moment in the
run_hrtimer_queues that turns interrupts on and releases the base lock.
In this time, a remove_hrtimer can be called while the state is still
HRTIMER_PENDING_CALLBACK, but it has been removed off the list.  The
remove_hrtimer will then try to remove this again.

Since I couldn't think of which state to use, I created the
HRTIMER_RUNNING, and used that instead.

I have a program (a simple jitter test) that, with out the patch,
reliably crashes the 2.6.15-rc5-rt1 on a slow UP machine.  With the
patch, it runs solidly, so I know this is the reason for the crash.

-- Steve

Index: linux-2.6.15-rc5-rt1/kernel/hrtimer.c
===================================================================
--- linux-2.6.15-rc5-rt1.orig/kernel/hrtimer.c	2005-12-13 15:51:52.000000000 -0500
+++ linux-2.6.15-rc5-rt1/kernel/hrtimer.c	2005-12-13 15:53:36.000000000 -0500
@@ -846,6 +846,7 @@
 		data = timer->data;
 		set_curr_timer(base, timer);
 		list_del(&timer->list);
+		timer->state = HRTIMER_RUNNING;
 		spin_unlock_irq(&base->lock);
 
 		/*
@@ -904,6 +905,7 @@
 		data = timer->data;
 		set_curr_timer(base, timer);
 		__remove_hrtimer(timer, base);
+		timer->state = HRTIMER_RUNNING;
 		spin_unlock_irq(&base->lock);
 
 		/*
Index: linux-2.6.15-rc5-rt1/include/linux/hrtimer.h
===================================================================
--- linux-2.6.15-rc5-rt1.orig/include/linux/hrtimer.h	2005-12-13 15:51:40.000000000 -0500
+++ linux-2.6.15-rc5-rt1/include/linux/hrtimer.h	2005-12-13 15:55:21.000000000 -0500
@@ -41,6 +41,7 @@
 enum hrtimer_state {
 	HRTIMER_INACTIVE,		/* Timer is inactive */
 	HRTIMER_EXPIRED,		/* Timer is expired */
+	HRTIMER_RUNNING,		/* Timer is currently running */
 	HRTIMER_PENDING,		/* Timer is pending */
 	HRTIMER_PENDING_CALLBACK,	/* Timer is expired, callback pending */
 };