ddi_enter_critical, ddi_exit_critical - enter and exit a critical
region of control
unsigned int ddi_enter_critical(void);
void ddi_exit_critical(unsignedint ddic);
Solaris DDI specific (Solaris DDI).
The returned value from the call to ddi_enter_critical() must
be passed to ddi_exit_critical().
Nearly all driver operations can be done without any special synchro‐
nization and protection mechanisms beyond those provided by, for exam‐
ple, mutexes (see mutex(9F)). However, for certain devices there can
exist a very short critical region of code which must be allowed to run
uninterrupted. The function ddi_enter_critical() provides a mechanism
by which a driver can ask the system to guarantee to the best of its
ability that the current thread of execution will neither be preempted
nor interrupted. This stays in effect until a bracketing call to
ddi_exit_critical() is made (with an argument which was the returned
value from ddi_enter_critical()).
The driver may not call any functions external to itself in between the
time it calls ddi_enter_critical() and the time it calls ddi_exit_crit‐
The ddi_enter_critical() function returns an opaque unsigned integer
which must be used in the subsequent call to ddi_exit_critical().
This function can be called from user, interrupt, or kernel context.
Driver writers should note that in a multiple processor system this
function does not temporarily suspend other processors from executing.
This function also cannot guarantee to actually block the hardware from
doing such things as interrupt acknowledge cycles. What it can do is
guarantee that the currently executing thread will not be preempted.
Do not write code bracketed by ddi_enter_critical() and ddi_exit_crit‐
ical() that can get caught in an infinite loop, as the machine may
crash if you do.
Writing Device Drivers
Jan 16, 2006 DDI_ENTER_CRITICAL(9F)