Writing Device Drivers

Mutual Exclusion Locks

In the SunOS 4.1 system, a driver had to be careful when accessing data shared between the tophalf and the interrupt routine. Because the interrupt could occur asynchronously, the interrupt routine could corrupt data or simply hang. To prevent this, portions of the top half of the driver would raise, using the various spl routines, the interrupt priority level of the CPU to block the interrupt from being handled:

	s = splr(pritospl(6));
 	/* access shared data */

In the SunOS 5.7 system, this no longer works. Changing the interrupt priority level of one CPU does not necessarily prevent another CPU from handling the interrupt. Also, two top-half routines may be running simultaneously with the interrupt running on a third CPU.

To solve this problem, the SunOS 5.7 system provides:

  1. A uniform module of execution--even interrupts run as threads. This blurs the distinction between the tophalf and the bottomhalf, as effectively every routine is a bottomhalf routine.

  2. A number of locking mechanisms-a common mechanism is to use mutual exclusion locks (mutexes):

 	/* access shared data */

A subtle difference from the SunOS 4.1 system is that, because everything is run by kernel threads, the interrupt routine needs to explicitly acquire and release the mutex. In the SunOS 4.1 system, this was implicit since the interrupt handler automatically ran at an elevated priority.

See "Multithreading Additions to the State Structure" for more information on locking.