ddi_enter_critical man page on SmartOS

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       ddi_enter_critical,  ddi_exit_critical  -  enter	 and  exit  a critical
       region of control

       #include <sys/conf.h>
       #include <sys/ddi.h>
       #include <sys/sunddi.h>

       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)

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