|
|
|
Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 469 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 469
438
DEADLOCKS
CHAP. 6
semaphores are all initialized to 1.
Mutexes can be used equally well.
The three
steps listed above are then implemented as a
down
on the semaphore to acquire the
resource, the use of the resource, and finally an
up
on the resource to release it.
These steps are shown in Fig. 6-1(a).
typedef int semaphore;
typedef int semaphore;
semaphore resource
1;
semaphore resource
1;
semaphore resource
2;
void process
A(void) {
void process
A(void) {
down(&resource
1);
down(&resource
1);
use
resource
1( );
down(&resource
2);
up(&resource
1);
use
both
resources( );
}
up(&resource
2);
up(&resource
1);
}
(a)
(b)
Figure 6-1.
Using a semaphore to protect resources. (a) One resource. (b) Two resources.
Sometimes processes need two or more resources.
They can be acquired se-
quentially, as shown in Fig. 6-1(b). If more than two resources are needed, they
are just acquired one after another.
So far, so good. As long as only one process is involved, everything works
fine. Of course, with only one process, there is no need to formally acquire re-
sources, since there is no competition for them.
Now let us consider a situation with two processes,
A
and
B
, and two re-
sources. Two scenarios are depicted in Fig. 6-2. In Fig. 6-2(a), both processes ask
for the resources in the same order.
In Fig. 6-2(b), they ask for them in a different
order.
This difference may seem minor, but it is not.
In Fig. 6-2(a), one of the processes will acquire the first resource before the
other one.
That process will then successfully acquire the second resource and do
its work. If the other process attempts to acquire resource 1 before it has been re-
leased, the other process will simply block until it becomes available.
In Fig. 6-2(b), the situation is different. It might happen that one of the proc-
esses acquires both resources and effectively blocks out the other process until it is
done. However, it might also happen that process
A
acquires resource 1 and proc-
ess
B
acquires resource 2.
Each one will now block when trying to acquire the
other one.
Neither process will ever run again. Bad news: this situation is a dead-
lock.
Here we see how what appears to be a minor difference in coding style—
which resource to acquire first—turns out to make the difference between the pro-
gram working and the program failing in a hard-to-detect way.
Because deadlocks
can occur so easily, a lot of research has gone into ways to deal with them.
This
chapter discusses deadlocks in detail and what can be done about them.
Ace your assessments! Get Better Grades
Browse thousands of Study Materials & Solutions from your Favorite Schools
Concordia University
Concordia_University
School:
Operating_Systems
Course:
Great resource for chem class. Had all the past labs and assignments
Leland P.
Santa Clara University
Introducing Study Plan
Using AI Tools to Help you understand and remember your course concepts better and faster than any other resource.
Find the best videos to learn every concept in that course from Youtube and Tiktok without searching.
Save All Relavent Videos & Materials and access anytime and anywhere
Prepare Smart and Guarantee better grades
Know more
Students also viewed documents
lab 18.docx
lab_18.docx
Course
Course
3
Module5QuizSTA2023.d...
Module5QuizSTA2023.docx.docx
Course
Course
10
Week 7 Test Math302....
Week_7_Test_Math302.docx.docx
Course
Course
30
Chapter 1 Assigment ...
Chapter_1_Assigment_Questions.docx.docx
Course
Course
5
Week 4 tests.docx.do...
Week_4_tests.docx.docx
Course
Course
23
Week 6 tests.docx.do...
Week_6_tests.docx.docx
Course
Course
106
