1. Review: Readers-Writers Problem
Multiple threads reading/writing a database
Many threads can read simultaneously
Only one can be writing at any time
When a writer is executing, nobody else can read or write

2. Review: Readers-Writers
While (1) {
generate_data();
down(database);
write();
up(database); }
READER:
While (1) {
down(protector);
rc++;
if (rc == 1) //first reader
down(database);
up(protector);
read();
rc--;
If (rc == 0) then // last one
up(database); …. }
Two semaphores:
database
protector
Can we replace database with a mutex?
Initial: protector=1, database =1
rc =0

3. Lecture 17: Deadlock: Conditions, Detection and Avoidance

4. Review Two Examples (Deadlock) Multiple Mutex Locks
Dining Philosopher Problem

5. Review: Taking Multiple Locks
proc1( ) {
pthread_mutex_lock(&m1);
/* use object 1 */
pthread_mutex_lock(&m2);
/* use objects 1 and 2 */
pthread_mutex_unlock(&m2);
pthread_mutex_unlock(&m1); }
proc2( ) {
pthread_mutex_lock(&m2);
/* use object 2 */
pthread_mutex_lock(&m1);
/* use objects 1 and 2 */
pthread_mutex_unlock(&m1);
pthread_mutex_unlock(&m2); }
In this example our threads are using two mutexes to control access to two different objects. Thread 1, executing proc1, first takes mutex 1, then, while still holding mutex 1, obtains mutex 2. Thread 2, executing proc2, first takes mutex 2, then, while still holding mutex 2, obtains mutex 1. However, things do not always work out as planned. If thread 1 obtains mutex 1 and, at about the same time, thread 2 obtains mutex 2, then if thread 1 attempts to take mutex 2 and thread 2 attempts to take mutex 1, we have a deadlock.
thread a
thread b

6. DEADLOCK

7. Roadmap for this week
Deadlocks
Strategies of dealing with deadlocks

8. In this lecture
Introduction to deadlocks

9. Resources Examples of computer resources
printers
tape drives
tables
Processes need access to resources in reasonable order
Suppose a process holds resource A and requests resource B
at same time another process holds B and requests A
both are blocked and remain so

10. Resources (1) Deadlocks occur when … Preemptable resources
processes are granted exclusive access to devices
we refer to these devices generally as resources
Preemptable resources
can be taken away from a process with no ill effects
Nonpreemptable resources
will cause the process to fail if taken away

11. Resources (2) Sequence of events required to use a resource
request the resource
use the resource
release the resource
Must wait if request is denied
requesting process may be blocked
may fail with error code

12. Introduction to Deadlocks
Formal definition : A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause
Usually the event is release of a currently held resource
None of the processes can …
run
release resources
be awakened

13. Four Conditions for Deadlock
Mutual exclusion condition
each resource assigned to 1 process or is available
Hold and wait condition
process holding resources can request additional
No preemption condition
previously granted resources cannot forcibly taken away
Circular wait condition
must be a circular chain of 2 or more processes
each is waiting for resource held by next member of the chain

14. Deadlock Modeling (1) Modeled with directed graphs
resource R assigned to process A
process B is requesting/waiting for resource S
process C and D are in deadlock over resources T and U

15. Deadlock Modeling (2)
A B C
How deadlock occurs