ITHREAD(9) FreeBSD Kernel Developer's Manual ITHREAD(9)


ithread_add_handler, ithread_create, ithread_destroy, ithread_priority, ithread_remove_handler, ithread_schedulekernel interrupt threads


#include < sys/param.h>
#include < sys/bus.h>
#include < sys/interrupt.h>

ithread_add_handler( struct ithd *ithread, const char *name, driver_intr_t handler, void *arg, u_char pri, enum intr_type flags, void **cookiep);

ithread_create( struct ithd **ithread, int vector, int flags, void (*disable)(int), void (*enable)(int), const char *fmt, ...);

ithread_destroy( struct ithd *ithread);

ithread_priority( enum intr_type flags);

ithread_remove_handler( void *cookie);

ithread_schedule( struct ithd *ithread, int do_switch);


Interrupt threads are kernel threads that run a list of handlers when triggered by either a hardware or software interrupt. Each interrupt handler has a name, handler function, handler argument, priority, and various flags. Each interrupt thread maintains a list of handlers sorted by priority. This results in higher priority handlers being executed prior to lower priority handlers. Each thread assumes the priority of its highest priority handler for its process priority, or PRIO_MAX if it has no handlers. Interrupt threads are also associated with a single interrupt source, represented as a vector number.

The ithread_create() function creates a new interrupt thread. The ithread argument points to an struct ithd pointer that will point to the newly created thread upon success. The vector argument specifies the interrupt source to associate this thread with. The flags argument is a mask of properties of this thread. The only valid flag currently for ithread_create() is IT_SOFT to specify that this interrupt thread is a software interrupt. The enable and disable arguments specify optional functions used to enable and disable this interrupt thread's interrupt source. The functions receive the vector corresponding to the thread's interrupt source as their only argument. The remaining arguments form a printf(9) argument list that is used to build the base name of the new ithread. The full name of an interrupt thread is formed by concatenating the base name of an interrupt thread with the names of all of its interrupt handlers.

The ithread_destroy() function destroys a previously created interrupt thread by releasing its resources and arranging for the backing kernel thread to terminate. An interrupt thread can only be destroyed if it has no handlers remaining.

The ithread_add_handler() function adds a new handler to an existing interrupt thread specified by ithread. The name argument specifies a name for this handler. The handler and arg arguments provide the function to execute for this handler and an argument to pass to it. The pri argument specifies the priority of this handler and is used both in sorting it in relation to the other handlers for this thread and to specify the priority of the backing kernel thread. The flags argument can be used to specify properties of this handler as defined in < sys/bus.h>. If cookiep is not NULL, then it will be assigned a cookie that can be used later to remove this handler.

The ithread_remove_handler() removes a handler from an interrupt thread. The cookie argument specifies the handler to remove from its thread.

The ithread_schedule() function schedules an interrupt thread to run. If the do_switch argument is non-zero and the interrupt thread is idle, then a context switch will be forced after putting the interrupt thread on the run queue.

The ithread_priority() function translates the INTR_TYPE_* interrupt flags into interrupt handler priorities.

The interrupt flags not related to the type of a particular interrupt ( INTR_TYPE_*) can be used to specify additional properties of both hardware and software interrupt handlers. The INTR_EXCL flag specifies that this handler cannot share an interrupt thread with another handler. The INTR_MPSAFE flag specifies that this handler is MP safe in that it does not need the Giant mutex to be held while it is executed. The INTR_ENTROPY flag specifies that the interrupt source this handler is tied to is a good source of entropy, and thus that entropy should be gathered when an interrupt from the handler's source triggers. Presently, the INTR_ENTROPY flag is not valid for software interrupt handlers.

It is not permitted to sleep in an interrupt thread; hence, any memory or zone allocations in an interrupt thread should be specified with the M_NOWAIT flag set. Any allocation errors must be handled thereafter.


The ithread_add_handler(), ithread_create(), ithread_destroy(), ithread_remove_handler(), and ithread_schedule() functions return zero on success and non-zero on failure. The ithread_priority() function returns a process priority corresponding to the passed in interrupt flags.


The swi_add() function demonstrates the use of ithread_create() and ithread_add_handler().

swi_add(struct ithd **ithdp, const char *name, driver_intr_t handler, 
     void *arg, int pri, enum intr_type flags, void **cookiep) 
 struct proc *p; 
 struct ithd *ithd; 
 int error; 
 if (flags & INTR_ENTROPY) 
  return (EINVAL); 
 ithd = (ithdp != NULL) ? *ithdp : NULL; 
 if (ithd != NULL) { 
  if ((ithd->it_flags & IT_SOFT) == 0) 
 } else { 
  error = ithread_create(&ithd, pri, IT_SOFT, NULL, NULL, 
      "swi%d:", pri); 
  if (error) 
   return (error); 
  if (ithdp != NULL) 
   *ithdp = ithd; 
 return (ithread_add_handler(ithd, name, handler, arg, pri + PI_SOFT, 
      flags, cookiep)); 


The ithread_add_handler() function will fail if:
Any of the ithread, handler, or name arguments are NULL.
The INTR_EXCL flag is specified and the interrupt thread ithread already has at least one handler, or the interrupt thread ithread already has an exclusive handler.
Could not allocate needed memory for this handler.

The ithread_create() function will fail if:

The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC.
A flag other than IT_SOFT was specified in the flags parameter.
Could not allocate needed memory for this interrupt thread.

The ithread_destroy() function will fail if:

The ithread argument is NULL.
The interrupt thread pointed to by ithread has at least one handler.

The ithread_remove_handler() function will fail if:

The cookie argument is NULL.

The ithread_schedule() function will fail if:

The ithread argument is NULL.
The interrupt thread pointed to by ithread has no interrupt handlers.


Interrupt threads and their corresponding API first appeared in FreeBSD 5.0.


Currently struct ithd represents both an interrupt source and an interrupt thread. There should be a separate struct isrc that contains a vector number, enable and disable functions, etc. that an ithread holds a reference to.
August 25, 2006 FreeBSD