Hardware Reference
In-Depth Information
normal execution
critical section
ceiling blocking
τ
1
A
B
direct blocking
τ
2
C
τ
3
C
C
B
B
C
p
3
P
1
P
2
P
3
t
0
t
1
t
2
t
3
t
4
t
5
t
6
t
7
t
8
t
9
Figure 7.14
Example of Priority Ceiling Protocol.
At time
t
8
,
τ
3
exits its outer critical section, unlocks
S
C
, and, since
τ
2
is awak-
ened,
τ
3
returns to its nominal priority
P
3
.
At this point,
τ
2
preempts
τ
3
and
executes until completion.
At time
t
9
,
τ
2
is completed; thus,
τ
3
resumes its execution.
Note that the Priority Ceiling Protocol introduces a third form of blocking, called
ceiling blocking
, in addition to direct blocking and push-through blocking caused by
the Priority Inheritance Protocol. This is necessary for avoiding deadlock and chained
blocking. In the previous example, a ceiling blocking is experienced by task
τ
1
at time
t
5
.
7.7.2
PROPERTIES OF THE PROTOCOL
The main properties of the Priority Ceiling Protocol are presented in this section. They
are used to analyze the schedulability and compute the maximum blocking time of
each task.
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