Introduction
Spin locks are a very simple and, in the right context, efficient method of
synchronising access to shared resources. The overhead of using Critical Sections or
Events for resource acquision is relatively large, but in contrast, a spin lock is
very quick.
Algorithm
Predictably, the algorithm for a spin lock is very simple. Each thread will
instantiate a CSpinLock with a shared lock variable. When a shared
resource is required, the threads call lock() and when the resource
has been finished with, call unlock().
lock() will cause the thread to loop until the resource is
available. The availability of the resource is defined by the value of shared
variable. If the value of this variable is zero, then the resource is available,
otherwise it is in use by another thread. When the resource is locked, the
shared variable holds the value 0x6b636f6c. This will read "lock" if it is cast to
a char *
Implementation
The CSpinLock class implements a spin lock synchronisation object.
Methods
explicit CSpinLock(long *plock = 0)
This is the only class constructor. It passes a pointer to a shared variable to hold
the lock value. This parameter can be omitted, in which case an internal class
lock variable will be used.
~CSpinLock()
Class destructor. Simply asserts that the object that is going out of scope
does not hold the lock.
void lock(void) const
Call this method to acquire a lock to the shared resource.
void unlock(void) const
Call this method to release the lock to the shared resource.
bool has_lock(void) const
Returns true/false to signify if the object currently holds the lock
Usage
The only decision to be made in using this class is in how the lock variable
is used. The easiest use is to omit the ctor parameter and let the class handle
this itself. Each resource that is to be shared can be synchronised using a
different template parameter value. Each thread can independently instantiate a
CSpinLock object and the class implementation will ensure that the
resource is synchronised correctly.
Contact
Web Site: http://homepage.virgin.net/cdm.henderson
e-mail: cdm.henderson@virgin.net
Revision History
17 Jun 2002 - Initial Revision
