When setting OS_CFG_ISR_POST_DEFERRED_EN to DEF_ENABLED, µC/OS-III locks the scheduler before entering a critical section and unlocks the scheduler when leaving the critical section.
OS_CRITICAL_ENTER() simply increments OSSchedLockNestingCtr to lock the scheduler. This is the variable the scheduler uses to determine whether or not the scheduler is locked. It is locked when the value is non-zero.
OS_CRITICAL_EXIT() decrements OSSchedLockNestingCtr and when the value reaches zero, invokes the scheduler.
OS_CRITICAL_EXIT_NO_SCHED() also decrements OSSchedLockNestingCtr, but does not invoke the scheduler when the value reaches zero.
The code for the macros is shown in the listing below: