Critical Regions

A critical region is a simple mechanism that prevents multiple threads from accessing at once code protected by the same critical region. More...

Collaboration diagram for Critical Regions:

Functions
void	critical_new (t_critical *x)
	Create a new critical region. More...
void	critical_enter (t_critical x)
	Enter a critical region. More...
void	critical_exit (t_critical x)
	Leave a critical region. More...
void	critical_free (t_critical x)
	Free a critical region created with critical_new(). More...
short	critical_tryenter (t_critical x)
	Try to enter a critical region if it is not locked. More...

Variables
BEGIN_USING_C_LINKAGE typedef pthread_mutex_t *	t_critical
	a critical region

Detailed Description

A critical region is a simple mechanism that prevents multiple threads from accessing at once code protected by the same critical region.

The code fragments could be different, and in completely different modules, but as long as the critical region is the same, no two threads should call the protected code at the same time. If one thread is inside a critical region, and another thread wants to execute code protected by the same critical region, the second thread must wait for the first thread to exit the critical region. In some implementations a critical region can be set so that if it takes too long for the first thread to exit said critical region, the second thread is allowed to execute, dangerously and potentially causing crashes. This is the case for the critical regions exposed by Max and the default upper limit for a given thread to remain inside a critical region is two seconds. Despite the fact that there are two seconds of leeway provided before two threads can dangerously enter a critical region, it is important to only protect as small a portion of code as necessary with a critical region.

Under Max 4.1 and earlier there was a simple protective mechanism called "lockout" that would prevent the scheduler from interrupting the low priority thread during sensitive operations such as sending data out an outlet or modifying members of a linked list. This lockout mechanism has been deprecated, and under the Mac OS X and Windows XP versions (Max 4.2 and later) does nothing. So how do you protect thread sensitive operations? Use critical regions (also known as critical sections). However, it is very important to mention that all outlet calls are now thread safe and should never be contained inside a critical region. Otherwise, this could result in serious timing problems. For other tasks which are not thread safe, such as accessing a linked list, critical regions or some other thread protection mechanism are appropriate.

In Max, the critical_enter() function is used to enter a critical region, and the critical_exit() function is used to exit a critical region. It is important that in any function which uses critical regions, all control paths protected by the critical region, exit the critical region (watch out for goto or return statements). The critical_enter() and critical_exit() functions take a critical region as an argument. However, for almost all purposes, we recommend using the global critical region in which case this argument is zero. The use of multiple critical regions can cause problems such as deadlock, i.e. when thread #1 is inside critical region A waiting on critical region B, but thread #2 is inside critical region B and is waiting on critical region A. In a flexible programming environment such as Max, deadlock conditions are easier to generate than you might think. So unless you are completely sure of what you are doing, and absolutely need to make use of multiple critical regions to protect your code, we suggest you use the global critical region.

In the following example code we show how one might use critical regions to protect the traversal of a linked list, testing to find the first element whose values is equal to "val". If this code were not protected, another thread which was modifying the linked list could invalidate assumptions in the traversal code.

critical_enter(0);
for (p = head; p; p = p->next) {
    if (p->value == val)
        break;
}
critical_exit(0);
return p;

And just to illustrate how to ensure a critical region is exited when multiple control paths are protected by a critical region, here's a slight variant.

critical_enter(0);
for (p = head; p; p = p->next) {
    if (p->value == val) {
        critical_exit(0);
        return p;
    }
}
critical_exit(0);
return NULL;

For more information on multi-threaded programming, hardware interrupts, and related topics, we suggest you perform some research online or read the relevant chapters of "Modern Operating Systems" by Andrew S. Tanenbaum (Prentice Hall). At the time of writing, some relevant chapters from this book are available for download in PDF format on Prentice Hall’s web site. See:

http://www.prenhall.com/divisions/esm/app/author_tanenbaum/custom/mos2e/

Look under "sample sections".

Function Documentation

critical_enter()

void critical_enter

(

t_critical

x

)

Enter a critical region.

Typically you will want the argument to be zero to enter the global critical region, although you could pass your own critical created with critical_new(). It is important to try to keep the amount of code in the critical region to a minimum. Exit the critical region with critical_exit().

Parameters

x	A pointer to a t_critical struct, or zero to uses Max’s global critical region.

See also

critical_exit()

Referenced by jit_global_critical_enter().

critical_exit()

void critical_exit

(

t_critical

x

)

Leave a critical region.

Typically you will want the argument to be zero to exit the global critical region, although, you if you are using your own critical regions you will want to pass the same one that you previously passed to critical_enter().

Parameters

x	A pointer to a t_critical struct, or zero to uses Max’s global critical region.

Referenced by jit_global_critical_exit().

critical_free()

void critical_free

(

t_critical

x

)

Free a critical region created with critical_new().

If you created your own critical region, you will need to free it in your object’s free method.

Parameters

x	The t_critical struct that will be freed.

critical_new()

void critical_new

(

t_critical *

x

)

Create a new critical region.

Normally, you do not need to create your own critical region, because you can use Max’s global critical region. Only use this function (in your object’s instance creation method) if you are certain you are not able to use the global critical region.

Parameters

x	A t_critical struct will be returned to you via this pointer.

critical_tryenter()

short critical_tryenter

(

t_critical

x

)

Try to enter a critical region if it is not locked.

Parameters

x	A pointer to a t_critical struct, or zero to uses Max’s global critical region.

Returns

returns non-zero if there was a problem entering

See also

critical_enter()