2.1 Conditions

This problem occurs if the following five conditions are satisfied:

(1) The attribute of the mailbox taken as an argument by a vref_mbx system call is TA_TPRI.

(2) An interrupt is requested during processing the vref_mbx system call.

(3) The handler invoked by the interrupt in (2) issues an ichg_pri system call.

(4) The ichg_pri system call alters a task's priority from low to high.

(5) The mailbox ID of the mailbox for which the task whose priority was changed in (4) is waiting and the one that is taken as an argument by the vref_mbx system call in (1) are the same.

    Example 1: Disabling and enabling interrupts by using loc_cpu() 
               and unl_cpu() respectively
     ---------------------------------------------------------------------
     void task(stacd)
     {
                     :
                 loc_cpu();        /* Disables interrupts */
          ercd = vref_mbx(&rmbx,ID_mbx);
                 unl_cpu();        /* Enables interrups */
                    :
     }
     ---------------------------------------------------------------------
    Example 2: Disabling and enabling interrupts by using the asm function
     ---------------------------------------------------------------------
     void int_handler(void)
     {
                    :
            /* Disables interrupts */

              asm(" mvfc      R0,PSW\n"
                  " and3      R0,R0,#0xFFBF\n"
                  " mvtc      R0,PSW\n");

          ercd = vref_mbx(&rmbx,ID_mbx);

            /* Enables interrupts */
              asm(" mvfc      R0,PSW\n"
                  " or3       R0,R0,#0x0040\n"
                  " mvtc      R0,PSW\n");
                    :
     }
     ---------------------------------------------------------------------