On a multiprocessor system, timer request blocks and watchdog timer structures could be accessed simultaneously by several processors. The kernel services shown below potentially alter critical information in these blocks and structures, and therefore check whether it is safe to perform the requested service before proceeding:
| tstop | Cancels a pending timer request. |
| w_clear | Removes a watchdog timer from the list of watchdog timers known to the kernel. |
| w_init | Registers a watchdog timer with the kernel. |
If the requested service cannot be performed, the kernel service returns an error value.
In order to be multiprocessor safe, the caller must check the value returned by these kernel services. If the service was not successful, the caller must take an appropriate action, for example, retrying in a loop. If the caller holds a device driver lock, it should release and then reacquire the lock within this loop in order to avoid deadlock.
Drivers which were written for uniprocessor systems do not check the return values of these kernel services and are not multiprocessor-safe. Such drivers can still run as funnelled device drivers.