Real-Time Preemption V0.7.52-07: rt_init_MUTEX_LOCKED declaration

This patch fixes broken rt_init_MUTEX_LOCKED declaration using rt_sema_init() macro. This way we fix a potential compile bug: rt_init_MUTEX_LOCKED calls there_is_no_init_MUTEX_LOCKED_for_RT_semaphores, which is not referenced. (e.g. drivers/char/watchdog/cpu5wdt.c: "cpu5wdt: Unknown symbol there_is_no_init_MUTEX_LOCKED_for_RT_semaphores")

--- realtime-preempt-2.6.13-rc4-RT-V0.7.52-07.orig 2005-07-31 23:35:18.000000000
+0000
+++ realtime-preempt-2.6.13-rc4-RT-V0.7.52-07 2005-08-01 15:59:31.000000000 +0000
@@ -8342,16 +8342,6 @@ Index: linux/drivers/cpufreq/cpufreq.c
===================================================================
--- linux.orig/drivers/cpufreq/cpufreq.c
+++ linux/drivers/cpufreq/cpufreq.c
-@@ -605,7 +605,8 @@ static int cpufreq_add_dev (struct sys_d
- policy->cpu = cpu;
- policy->cpus = cpumask_of_cpu(cpu);
-
-- init_MUTEX_LOCKED(&policy->lock);
-+ init_MUTEX(&policy->lock);
-+ down(&policy->lock);
- init_completion(&policy->kobj_unregister);
- INIT_WORK(&policy->update, handle_update, (void *)(long)cpu);
-
@@ -614,6 +615,7 @@ static int cpufreq_add_dev (struct sys_d
*/
ret = cpufreq_driver->init(policy);
@@ -14350,7 +14340,7 @@ Index: linux/include/linux/rt_lock.h
===================================================================
--- /dev/null
+++ linux/include/linux/rt_lock.h
-@@ -0,0 +1,391 @@
+@@ -0,0 +1,385 @@
+#ifndef __LINUX_RT_LOCK_H
+#define __LINUX_RT_LOCK_H
+
@@ -14589,14 +14579,8 @@ Index: linux/include/linux/rt_lock.h
+extern void FASTCALL(__init_MUTEX(struct semaphore *sem, char *name, char
*file, int line));
+#define rt_init_MUTEX(sem) \
+ __init_MUTEX(sem, #sem, __FILE__, __LINE__)
-+
-+extern void there_is_no_init_MUTEX_LOCKED_for_RT_semaphores(void); -+
-+/*
-+ * No locked initialization for RT semaphores
-+ */
+#define rt_init_MUTEX_LOCKED(sem) \
-+ there_is_no_init_MUTEX_LOCKED_for_RT_semaphores()
++ rt_sema_init(sem, 0)
+extern void FASTCALL(rt_down(struct semaphore *sem));
+extern int FASTCALL(rt_down_interruptible(struct semaphore *sem)); +extern int FASTCALL(rt_down_trylock(struct semaphore *sem));

Regards,

					Luca

Ingo,

When did you solve the problem of ownership of locked semaphores? ;-)

Luca,
Unless Ingo did solve the problem of semaphores that can be locked by one task and unlocked by another task, I wouldn't use your patch. There's a problem with priority inheritance when it comes to these semaphores. That is who owns a locked semaphore that will later be unlocked by someone else? When a RT process blocks on this semaphore, who does it boost to release it? Ingo purposely put this in to crash the compile so that we know where this can be a problem right away.

The patch you wanted to send was:

Index: linux_realtime_ernie/drivers/char/watchdog/cpu5wdt.c
===================================================================
@@ -56,7 +56,7 @@
 /* some device data */
 
 static struct {
-	struct semaphore stop;
+	struct compat_semaphore stop;
 	volatile int running;
 	struct timer_list timer;
 	volatile int queue;

And it works nice ;)

Well it's not really a counter. It's one of these situations where the semaphore is locked on init, later someone else ups it on a trigger, and then the most horrible, it gets down again on module exit! I'm not really sure what it is doing. The only place that an up is called is on the watchdog trigger. I guess it's being used to not let you unload the module if the watchdog hasn't gone off yet, or something to that effect.

-- Steve

the right solution would be to mark policy->lock as a compat_semaphore. That will revert things back to the stock semantics. (at the price of not having PI, which isnt a big issue in this case.)

the reason for not allowing init_MUTEX_LOCKED() is that in basically every case that does it, the semaphore is not used as a true mutex in the strict sense. So the affected semaphore should be changed to compat_semaphore, to gain full semantics.

Ingo

I was looking at Luca's original message where he showed the bug of -- drivers/char/watchdog/cpu5wdt.c: "cpu5wdt: Unknown symbol there_is_no_init_MUTEX_LOCKED_for_RT_semaphores") -- Looking into this file the only init_MUTEX_LOCKED that I found was

static int __devinit cpu5wdt_init(void)

{
	unsigned int val;
	int err;

[...]

	/* watchdog reboot? */
	val = inb(port + CPU5WDT_STATUS_REG);
	val = (val >> 2) & 1;
	if ( !val )
		printk(KERN_INFO PFX "sorry, was my fault\n");

	init_MUTEX_LOCKED(&cpu5wdt_device.stop);
	cpu5wdt_device.queue = 0;

	clear_bit(0, &cpu5wdt_device.inuse);

Here I see that cpu5wdt_device.stop is being initialized with init_MUTEX_LOCKED, so that is what I went to fix. I even added the driver to my config and compiled it before sending it in. I don't have the device, but the driver compiled :-)

-- Steve

ok - the fix is in -52-10.

Ingo