BASIC CONCEPTS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 33

τ W τ D

shared
resource wait(s) wait(s)

critical
section

critical
section

R

signal(s) signal(s)

Figure 2.11 Structure of two tasks that share a mutually exclusive resource protected by

a semaphore.

blocked

plot(x,y)

τ

R

D

x=4 y=8

τ W

R R

t
(1,2) (4,2) (4,8)

Figure 2.12 Example of schedule when the resource is protected by a semaphore.

To avoid the problem illustrated in Figure 2.10, both tasks have to encapsulate the
instructions that manipulate the shared variables into a critical section. If a binary
semaphore s is used for this purpose, then each critical section must begin with a
wait(s) primitive and must end with a signal(s) primitive, as shown in Figure 2.11.

If the resource is free, the wait(s) primitive executed by τ W notifies that a task is using
the resource, which becomes locked until the task executes the signal(s) . Hence, if
τ D preempts τ W inside the critical section, it is blocked as soon as it executes wait(s) ,
and the processor is given back to τ W . When τ W exits its critical section by executing
signal(s) , then τ D is resumed and the processor is given to the ready task with the
highest priority. The resulting schedule is shown in Figure 2.12.