Re: Kernel Recursive mutexes 10.4.9 ++

• Subject: Re: Kernel Recursive mutexes 10.4.9 ++
• From: Terry Lambert <email@hidden>
• Date: Thu, 19 Jul 2007 00:26:00 -0700
• Delivered-to: email@hidden
• Delivered-to: email@hidden

I'd caution you on a couple of things:

(1)	your increments and decrements need to use atomic instructions

-- Terry

Terry,

OK, got the idea of that solution.

I found another reference to this issue, is it a good one and does it cover my needs?

>
> typedef struct pthread_mutex_recursive_np_t {
> pthread_mutex_t mutex;
> pthread_t owner;
> UInt32 count;
> } pthread_mutex_recursive_np_t;
>
> void pthread_mutex_lock_recursive_np(pthread_mutex_recursive_np_t *mutex)
> {
> pthread_t self;
>
> self = pthread_self();
> if (mutex->owner != self)
> {
> pthread_mutex_lock(&mutex->mutex);
> mutex->owner = self;
> mutex->count = 1;
> }
> else
> mutex->count += 1;
> }
>
> void pthread_mutex_unlock_recursive_np(pthread_mutex_recursive_np_t
> *mutex)
> {
> if (--mutex->count == 0)
> {
> mutex->owner = NULL;
> pthread_mutex_unlock(&mutex->mutex);
> }
> }

=======  Cheers, Erez

On Jul 19, 2007, at 10:01 AM, Terry Lambert wrote:

If funcB has to be callable unlocked somewhere, then:

funcA() {
	lock mutex
	funcB_locked()
	do stuff...
	unlock mutex
}

funcB() {
	lock mutex
	funcB_locked()
	unlock mutex
}

/* MUST be called with mutex held! */
funcB_locked()
{
	do stuff...
}

We do this sort of thing in the kernel in a number of places, where reference counts would be inappropriate.

-- Terry

On Jul 18, 2007, at 9:36 PM, Erez Kaplan wrote:

Terry,

Thanks for the explanation.

I am facing a some what strange sequence of calling (forced by PC porting) which looks something like this:
Currently I get a deadlock situation.
How would you outline handling this deadlock?

funcA() {

lock mutex
	funcB()
	do stuff...
unlock mutex
}

funcB() {

lock mutex
	do stuff...
unlock mutex
}

Cheers, Erez

On Jul 19, 2007, at 7:24 AM, Terry Lambert wrote:

On Jul 18, 2007, at 8:54 PM, Erez Kaplan wrote:

On Jul 18, 2007, at 10:00 PM, Terry Lambert wrote:

On Jul 18, 2007, at 2:37 AM, Erez Kaplan wrote:

Hi,

10.4.9+ 10.5
How do I obtain a Kernel Recursive mutexe?

The available api does not support this.
lck_mtx_alloc_init(gmutex_grp_m, LCK_ATTR_NULL)

You don't; this is what reference and use counts are for.

Can you explain a bit more or give reference to an example.

For example, all thread reentrant FSs will take an I/O reference on a vnode, and use that reference to prevent the FS from being unmounted out from under an operation in progress, rather than holding a lock over the complete operation (thus unnecessarily serializing all other I/O to the vnode). Here is a simplified design pattern:

some function(object)
	lock mutex
	atomic increment reference count
	unlock mutex

	... long duration operation ...

	lock mutex
	atomic decrement reference count
	unlock mutex

/* wake up destroy routine; destroy routine may fail acquisition race */
if !reference count
wakeup(object)

...

destroy(object)
	lock mutex
again:
	/* Don't destroy the object while someone is using it */
	if reference count
		msleep(mutex, object)
		goto again	/* avoid race */

	/* last reference gone, OK to kill it */
	destroy object
	unlock mutex

-- Terry

//======================
Erez Kaplan
Senior software engineer OSX
email@hidden
//======================

//======================
Erez Kaplan
Senior software engineer OSX
email@hidden
//======================

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