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Yet another synchronization problem The dining philosophers problem Deadlocks o Modeling deadlocks o Dealing with deadlocks Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 1

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Page 1: The dining philosophers problem Deadlocksos152/wiki.files/...Yet another synchronization problem The dining philosophers problem Deadlocks o Modeling deadlocks o Dealing with deadlocks

Yet another synchronization problem

The dining philosophers problem

Deadlocks

o Modeling deadlocks

o Dealing with deadlocks

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 1

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The Dining Philosophers Problem

Philosophers o think

o take forks (one at a time)

o eat

o put forks (one at a time)

Eating requires 2 forks

Pick one fork at a time

How to prevent deadlock?

What about starvation?

What about concurrency?

Slide taken from a presentation by Gadi Taubenfeld, IDC

2 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan

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Who are these (figure from wiki)?

3 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan

Plato

Socrates Voltaire

Descarte Confuzius

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Dining philosophers: definition

Each process needs two resources

Every pair of processes compete for a specific resource

A process may proceed only if it is assigned both resources

Every process that is waiting for a resource should sleep (be blocked)

Every process that releases its two resources must wake-up the two competing processes for these resources, if they are interested

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 4

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Slide taken from a presentation by Gadi Taubenfeld, IDC

An incorrect naïve solution

( means “waiting for this fork”)

5 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan

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Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan

The solution

o A philosopher first gets

o only then it tries to take the 2 forks.

Dining philosophers: textbook solution

Slide taken from a presentation by Gadi Taubenfeld, IDC

6

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Dining philosophers: textbook solution code

#define N 5

#define LEFT (i-1) % N

#define RIGHT (i+1) % N

#define THINKING 0

#define HUNGRY 1

#define EATING 2

int state[N];

semaphore mutex = 1;

semaphore s[N]; // per each philosopher

void philosopher(int i) {

while(TRUE) {

think();

pick_sticks(i);

eat();

put_sticks(i);

}

}

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 7

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Dining philosophers: textbook solution code Π

void pick_sticks(int i) {

down(&mutex);

state[i] = HUNGRY;

test(i);

up(&mutex);

down(&s[i]);

}

void put_sticks(int i) {

down(&mutex);

state[i] = THINKING;

test(LEFT);

test(RIGHT);

up(&mutex);

}

void test(int i) {

if(state[i] == HUNGRY && state[LEFT] != EATING && state[RIGHT] != EATING) {

state[i] = EATING;

up(&s[i]); }

}

Is the algorithm deadlock-free? What about starvation?

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 8

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Textbook solution code: starvation is possible

Eat

Eat

Block Starvation!

Slide taken from a presentation by Gadi Taubenfeld, IDC

9 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan

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Monitor-based implementation

monitor diningPhilosophers

condition self[N];

integer state[N];

procedure pick_sticks(i){

state[i] := HUNGRY;

test(i);

if state[i] <> EATING

then wait(self[i]);

}

procedure put_sticks(i){

state[i] := THINKING;

test(LEFT);

test(RIGHT);

procedure test(i){

if (state[LEFT] <> EATING &&

state[RIGHT] <> EATING &&

state[i] = HUNGRY)

then {

state[i] := EATING;

signal(self[i]);

}

}

for i := 0 to 4 do state[i] := THINKING;

end monitor

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Text-book solution disadvantages

An inefficient solution

o reduces to mutual exclusion

o not enough concurrency

o Starvation possible

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The LR Solution

If the philosopher acquires one fork and the other fork is not immediately available, she holds the acquired fork until the other fork is free.

Two types of philosophers:

o L -- The philosopher first obtains its left fork and then its right fork.

o R -- The philosopher first obtains its right fork and then its left fork.

The LR solution: the philosophers are assigned acquisition strategies as follows: philosopher i is R-type if i is even, L-type if i is odd.

R

L

R

L

R

L

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 12

Slide taken from a presentation by Gadi Taubenfeld, IDC

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Theorem: The LR solution is starvation-free

Assumption: “the fork is fair”.

( means “first fork taken”)

R

L

L

R R

L

0

1

2 3

4

6

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 13

Slide taken from a presentation by Gadi Taubenfeld, IDC

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Deadlocks

The dining philosophers problem

Deadlocks

o Modeling deadlocks

o Dealing with deadlocks

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie

Zivan

14

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Synchronization: Deadlocks

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Deadlocks

Deadlock of Resource Allocation: o Process A requests and gets Tape drive

o Process B requests and gets Fast Modem

o Process A requests Fast Modem and blocks

o Process B requests Tape drive and blocks

Deadlock situation: Neither process can make progress and no process can release its allocated device (resource)

Both resources (devices) require exclusive access

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 16

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Resources

Resources - Tapes, Disks, Printers, Database Records, semaphores, etc.

Some resources are non-preemptable (i.e. tape drive)

It is easier to avoid deadlock with preemptable resources (e.g., main memory, database records)

Resource allocation procedure o Request

o Use

o Release only at the end – and leave

Block process while waiting for Resources

Iterate

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 17

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Defining Deadlocks

A set of processes is deadlocked if each process is waiting for

an event that can only be caused by another process in the

set

Necessary conditions for deadlock:

1. Mutual exclusion: exclusive use of resources

2. Hold and wait: process can request resource while holding another

resource

3. No preemption: only holding process can release resource

4. Circular wait: there is an oriented circle of processes, each of which is

waiting for a resource held by the next in the circle

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Modeling deadlocks

modeled by a directed graph (resource graph)

o Requests and assignments as directed edges

o Processes and Resources as vertices

Cycle in graph means deadlock

Q

A

Process A holds resource Q

P

B

Process B requests

resource Q

R S

F

M

Deadlock

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Different possible runs: an example

A

R

B C

S T

A

Request R Request S Release R Release S

B

Request S Request T Release S Release T

C

Request T Request R Release T Release R

Round-robin scheduling:

1. A requests R

2. B requests S

3. C requests T

4. A requests S

5. B requests T

6. C requests R

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Different possible runs: an example

A

R

B C

S T

A

Request R Request S Release R Release S

B

Request S Request T Release S Release T

C

Request T Request R Release T Release R

An alternative scheduling:

1. A requests R

2. C requests T

3. A requests S

4. C requests R

5. A releases R

6. A releases S

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 21

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Multiple Resources of each Type

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A Directed Cycle But No Deadlock

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 23

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Resource Allocation Graph With A Deadlock

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 24

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Basic Facts

If graph contains no cycles no deadlock

If graph contains a cycle

o if only one instance per resource type, then deadlock

o if several instances per resource type, deadlock

possible

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 25

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Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 26

Dealing with Deadlocks

The dining philosophers problem

Deadlocks

o Modeling deadlocks

o Dealing with deadlocks

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Dealing with Deadlocks

Possible Strategies:

o Prevention structurally negate one of the four necessary conditions

o Avoidance allocate resources carefully, so as to avoid deadlocks

o Detection and recovery

o Do nothing (The “ostrich algorithm’’) deadlocks are rare and hard to tackle... do nothing

Example: Unix - process table with 1000 entries and 100 processes

each requesting 20 FORK calls... Deadlock.

users prefer a rare deadlock over frequent refusal of FORK

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Deadlock prevention

Attack one of the 4 necessary conditions:

1. Mutual exclusion o Minimize exclusive allocation of devices

o Use spooling: only spooling process requests access (not good for all devices - Tapes; Process Tables); may fill up spools (disk space deadlock)...

2. Hold and Wait o Request all resources immediately (before execution)

Problem: resources not known in advance, inefficient

or

o to get a new resource, free everything, then request everything again (including new resource)

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Attack one of the 4 necessary conditions (cont'd)

3. No preemption o Not always possible (e.g., printer, tape-drive)

4. Circular wait condition o Allow holding only a single resource (too restrictive)

o Number resources, allow requests only in ascending order:

Request only resources numbered higher than anything currently held

Impractical in general

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Deadlock Avoidance

System grants resources only if it is safe

basic assumption: maximum resources required by each process is known in advance

Safe state:

Not deadlocked

There is a scheduling that satisfies all possible future requests

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 30

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Safe states: example

p q l1 l2 l4 l3

l5

l6

l7

l8

r

s

t

A

B

Printer

Plotter

Plotter

Printer

Both have printer

Both have plotter

Both have both

Unsafe state

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 31

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Safe and Unsafe states (single resource)

Fig. 6-9. Three resource allocation states: (a) Safe. (b) Safe. (c) Unsafe.

(a) (b) (c)

Safe state: o Not deadlocked

o There is a way to satisfy all possible future requests

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 32

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Banker's Algorithm, Dijkstra 1965 (single resource)

Checks whether a state is safe 1. Pick a process that can terminate after fulfilling the rest of its requirements (enough free resources) 2. Free all its resources (simulation) 3. Mark process as terminated 4. If all processes marked, report “safe”, halt 5. If no process can terminate, report “unsafe”, halt 6. Go to step 1

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 33

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Multiple resources of each kind

Assume n processes and m resource classes

Use two matrixes and two vectors:

o Current allocation matrix Cn x m

o Request matrix Rn x m (remaining requests)

o Existing resources vector Em

o Available resources vector Am

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 34

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Banker’s Algorithm for multiple resources

1. Look for a row of R whose unmet resource needs are all smaller

than or equal to A. If no such row exists, the system will

eventually deadlock.

2. Otherwise, assume the process of the row chosen finishes

(which will eventually occur). Mark that process as terminated

and add the i’th row of C to the A vector

3. Repeat steps 1 and 2 until either all processes are marked

terminated, which means safe, or until a deadlock occurs,

which means unsafe.

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 35

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deadlock avoidance – an example with 4 resource types, 5 processes Tape-drives Plotters Scanners CD-ROMs

E = (6 3 4 2) A = (1 0 2 0)

C = R =

1 1 0 3 A

0 0 1 0 B

0 1 1 1 C

1 0 1 1 D

0 0 0 0 E

T P S C

0 0 1 1 A

2 1 1 0 B

0 0 1 3 C

0 1 0 0 D

0 1 1 2 E

T P S C

Is the current state safe? Yes, let’s see why…

We let D run until it finishes

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 36

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deadlock avoidance – an example with 4 resource types, 5 processes

Tape-drives Plotters Scanners CD-ROMs E = (6 3 4 2) A = (2 1 2 1)

C = R =

1 1 0 3 A

0 0 1 0 B

0 1 1 1 C

0 0 0 0 D

0 0 0 0 E

T P S C

0 0 1 1 A

2 1 1 0 B

0 0 1 3 C

0 0 0 0 D

0 1 1 2 E

T P S C

We now let E run until it finishes

Next we let A run until it finishes

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 37

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deadlock avoidance – an example with 4 resource types, 5 processes

Tape-drives Plotters Scanners CD-ROMs E = (6 3 4 2) A = (5 1 3 2)

C = R =

0 0 0 0 A

0 0 1 0 B

0 1 1 1 C

0 0 0 0 D

0 0 0 0 E

T P S C

0 0 0 0 A

2 1 1 0 B

0 0 1 3 C

0 0 0 0 D

0 0 0 0 E

T P S C

Finally we let B and C run.

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 38

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Back to original state

Tape-drives Plotters Scanners CD-ROMs E = (6 3 4 2) A = (1 0 2 0)

C = R =

1 1 0 3 A

0 0 1 0 B

0 1 1 1 C

1 0 1 1 D

0 0 0 0 E

T P S C

0 0 1 1 A

2 1 1 0 B

0 0 1 3 C

0 1 0 0 D

0 1 1 2 E

T P S C

If B now requests a Scanner, we can allow it.

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 39

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This is still a safe state…

Tape-drives Plotters Scanners CD-ROMs E = (6 3 4 2) A = (1 0 1 0)

C = R =

1 1 0 3 A

0 1 1 0 B

0 1 1 1 C

1 0 1 1 D

0 0 0 0 E

T P S C

0 0 1 1 A

2 0 1 0 B

0 0 1 3 C

0 1 0 0 D

0 1 1 2 E

T P S C

If E now requests a Scanner, granting the request leads to an unsafe state

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This state is unsafe

Tape-drives Plotters Scanners CD-ROMs E = (6 3 4 2) A = (1 0 0 0)

C = R =

1 1 0 3 A

0 1 1 0 B

0 1 1 1 C

1 0 1 1 D

0 1 0 0 E

T P S C

0 0 1 1 A

2 0 1 0 B

0 0 1 3 C

0 1 0 0 D

0 0 1 2 E

T P S C

We must not grant E’s request

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Deadlock Avoidance is not practical

Maximum resource request per process is unknown

beforehand

Resources may disappear

New processes (or resources) may appear

Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 42

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Deadlock Detection and Recovery

Find if a deadlock exists

if it does, find which processes and resources it involes

Detection: detect cycles in resource graph

Algorithm: DFS + node and arc marking

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Find cycles:

For each node, N, in the graph, perform the following 5 steps with N as the starting node

1. Initialize L to the empty list and designate all arcs as unmarked

2. Add the current node to the end of L and check if the node appears twice in L. If it does, the graph contains a cycle, terminate.

3. If there are any unmarked arcs from the given node, go to 4., if not go to

5.

4. Pick an unmarked outgoing arc and mark it. Follow it to the new current node and go to 2.

5. We have reached a deadend. Go back to the previous node, make it the current node and go to 3. If all arcs are marked and the node is the initial node, there are no cycles in the graph, terminate

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Detection - extract a cycle

1. Process A holds R and requests S

2. Process B holds nothing and requests T

3. Process C holds nothing and requests S

4. Process D holds U and requests S and T

5. Process E holds T and requests V

6. Process F holds W and requests S

7. Process G holds V and requests U

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When should the system check for deadlock ?

Whenever a request is made - too expensive

every k time units...

whenever CPU utilization drops bellow some threshold

(indication of a possible deadlock..)

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Recovery

Preemption - possible in some rare cases

temporarily take a resource away from its current owner

Rollback - possible with checkpointing

Keep former states of processes (checkpoints) to enable release of resources and going back

Killing a process - easy way out, may cause problems in some cases, depending on process being rerunable…

Bottom line: hard to recover from deadlock, avoid it

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Example - deadlocks in DBMSs

For database records that need locking first and

then updating (two-phase locking)

Deadlocks occur frequently because records are

dynamically requested by competing processes

DBMSs, therefore, need to employ deadlock

detection and recovery procedures

Recovery is possible - transactions are

“checkpointed” - release everything and restart

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Additional deadlock issues

Deadlocks may occur with respect to actions of processes, not resources - waiting for semaphores

Starvation can result from a bad allocation policy (such as smallest-file-first, for printing) and for the “starved” process will be equivalent to a deadlock (cannot finish running)

Summary of deadlock treatment:

o Ignore problem

o Detect and recover

o Avoid (be only in safe states)

o Prevent by using an allocation policy or conditions

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The Situation in Practice

Most OSs in use, and specifically Windoes, Linux…, ignore

deadlock or do not detect it

Tools to kill processes but usually without loss of data

In Windows NT there is a system call WaitForMultipleObjects

that requests all resources at once

o System provides all resources, if free

o There is no lock of resources if only few are free

o Prevents Hold & Wait, but difficult to implement!

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Linux: the Big Kernel Lock

Linux was first designed with Coarse-Grained Locking

The whole kernel was wrapped in a giant lock around it to avoid deadlocks (kernel / interrupt handlers / user threads) introduced in Linux 2.0.

Work began in 2008 to remove the big kernel lock: http://kerneltrap.org/Linux/Removing_the_Big_Kernel_Lock

It was carefully replaced with fine-grained locks until it was removed in Linux 2.6.39 (in 2011!) https://lwn.net/Articles/424657/

Operating Systems, 2013, Meni Adler, Michael Elhadad & Amnon Meisels 54 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 54

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Xv6 and deadlocks

Xv6 uses a few coarse data-structure specific locks; for example, xv6 uses a single lock protecting the process table and its invariants, which are described in Chapter 5.

A more fine-grained approach would be to have a lock per entry in the process table so that threads working on different entries in the process table can proceed in parallel.

However, it complicates operations that have invariants over the whole process table, since they might have to take out several locks.

To avoid such deadlocks, all code paths must acquire locks in the same order. Deadlock avoidance is another example illustrating why locks must be part of a function’s specification: the caller must invoke functions in a consistent order so that the functions acquire locks in the same order

Because xv6 uses coarse-grained locks and xv6 is simple, xv6 has few lock-order chains. The longest chain is only two deep. For example, ideintr holds the ide lock while calling wakeup, which acquires the ptable lock.

Operating Systems, 2013, Meni Adler, Michael Elhadad & Amnon Meisels 55 Operating Systems, 2015, Meni Adler, Danny Hendler & Roie Zivan 55