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Recall that one definition of an operating system is a resource allocator.There are many resources that can be allocated to only one process at atime, and we have seen several operating system features that allow this,such as mutexes, semaphores or file locks.

Sometimes a process has to reserve more than one resource. For example,a process which copies files from one tape to another generally requires two tapedrives. A process which deals with databases may need to lock multiplerecords in a database.

In general, resources allocated to a process are not preemptable;this means that once a resource has been allocated to a process, there isno simple mechanism by which the system can take the resource back fromthe process unless the process voluntarily gives it up or the systemadministrator kills the process.This can lead to a situation called deadlock. A set of processes orthreads is deadlocked when each process or thread is waiting for a resource tobe freed which is controlled by another process. Here is anexample of a situation where deadlock can occur.Suppose thread 1 is running and locks M1, but before it can lock M2, it is interrupted. Thread 2 starts running; it locks M2, when it tries to obtainand lock M1, it is blocked because M1 is already locked (by thread 1).Eventually thread 1 starts running again, and it tries to obtain and lockM2, but it is blocked because M2 is already locked by thread 2. Both threadsare blocked; each is waiting for an event which will never occur.

Traffic gridlock is an everyday example of a deadlock situation.

In order for deadlock to occur, four conditions must be true.

• Mutual exclusion - Each resource is either currently allocated to exactly oneprocess or it is available. (Two processes cannot simultaneously control thesame resource or be in their critical section).
• Hold and Wait - processes currently holding resources can request new resources
• No preemption - Once a process holds a resource, it cannot be taken away byanother process or the kernel.
• Circular wait - Each process is waiting to obtain a resource which isheld by another process.

The dining philosophers problem discussed in an earlier section is a classic example ofdeadlock. Each philosopher picks up his or her left fork and waits for the right forkto become available, but it never does.

Deadlock can be modeled with a directed graph. In a deadlock graph, vertices representeither processes (circles) or resources (squares). A process which has acquired a resourceis show with an arrow (edge) from the resource to the process. A process which hasrequested a resource which has not yet been assigned to it is modeled with an arrow fromthe process to the resource. If these create a cycle, there is deadlock.

The deadlock situation in the above code can be modeled like this.

This graph shows an extremely simple deadlock situation, but it is alsopossible for a more complex situation to create deadlock. Here is an exampleof deadlock with four processes and four resources.

There are a number of ways that deadlock can occur in an operating situation.We have seen some examples, here are two more.

• Two processes need to lock two files, the first process locks one filethe second process locks the other, and each waits for the other to free upthe locked file.
• Two processes want to write a file to a print spool area at the same timeand both start writing. However, the print spool area is of fixed size,and it fills up before either process finishes writing its file, so bothwait for more space to become available.

Solutions to deadlock

There are several ways to address the problem of deadlock in an operatingsystem.

• Just ignore it and hope it doesn't happen
• Detection and recovery - if it happens, take action
• Dynamic avoidance by careful resource allocation. Check to see if a resource can be granted, and if granting it will cause deadlock, don't grant it.
• Prevention - change the rules

The text refers to this as the Ostrich Algorithm. Just hope thatdeadlock doesn't happen. In general, this is a reasonable strategy. Deadlockis unlikely to occur very often; a system can run for years without deadlockoccurring. If the operating system has a deadlock prevention or detectionsystem in place, this will have a negative impact on performance (slowthe system down) because whenever a process or thread requests a resource,the system will have to check whether granting this request could cause apotential deadlock situation.

If deadlock does occur, it may be necessary to bring the system down, orat least manually kill a number of processes, but even that is not anextreme solution in most situations.

Deadlock detection and recovery

As we saw above, if there is only one instance of each resource, it ispossible to detect deadlock by constructing a resource allocation/requestgraph and checking for cycles. Graph theorists have developed a number ofalgorithms to detect cycles in a graph. The book discusses one of these.It uses only one data structure L a list of nodes.

A cycle detection algorithm

For each node N in the graph

1. Initialize L to the empty list and designate all edges as unmarked
2. Add the current node to L and check to see if it appears twice.If it does, there is a cycle in the graph.
3. From the given node, check to see if there are any unmarked outgoing edges.If yes, go to the next step, if no, skip the next step
4. Pick an unmarked edge, mark it, then follow it to the new currentnode and go to step 3.
5. We have reached a dead end. Go back to the previous node and makethat the current node. If the current node is the starting Node and thereare no unmarked edges, there are no cycles in the graph. Otherwise, goto step 3.

The algorithm needs to search each node; let's start at node P1. We addP1 to L and follow the only edge to R1, marking that edge. R1 is nowthe current node so we add that to L, checking to confirm that it is notalready in L. We then follow the unmarked edge to P2, marking the edge,and making P2 the current node. We add P2 to L, checking to make sure thatit is not already in L, and follow the edge to R2. This makes R2 the currentnode, so we add it to L, checking to make sure that it is not already there.We are now at a dead end so we back up, making P2 the current node again.There are no more unmarked edges from P2 so we back up yet again, makingR1 the current node. There are no more unmarked edges from R1 so weback up yet again, making P1 the current node. Since there are no moreunmarked edges from P1 and since this was our starting point, we arethrough with this node (and all of the nodes visited so far).

We move to the next unvisited node P3, and initialize L to empty.We first follow the unmarked edge to R1, putting R1 on L. Continuing, we make P2 the current node and then R2. Since weare at a dead end, we repeatedly back up until P3 becomes the currentnode again.

L now contains P3, R1, P2, and R2. P3 is the current node, and ithas another unmarked edge to R3. We make R3 the current node, add itto L, follow its edge to P4. We repeat this process, visitingR4, then P5, then R5, then P3. When we visit P3 again we note that it is already on L, so we have detected a cycle, meaning that thereis a deadlock situation.

Once deadlock has been detected, it is not clear what the systemshould do to correct the situation. There are three strategies.

• Preemption - we can take an already allocated resource awayfrom a process and give it to another process. This can presentproblems. Suppose the resource is a printer and a print job is half completed. It is often difficult to restart such a jobwithout completely starting over.
• Rollback - In situations where deadlock is a real possibility,the system can periodically make a record of the state of eachprocess and when deadlock occurs, roll everything back to the lastcheckpoint, and restart, but allocating resources differentlyso that deadlock does not occur. This means that all workdone after the checkpoint is lost and will have to be redone.
• Kill one or more processes - this is the simplest and crudest,but it works.

Deadlock avoidance

The above solution allowed deadlock to happen, then detected thatdeadlock had occurred and tried to fix the problem after the fact.Another solution is to avoid deadlock by only granting resourcesif granting them cannot result in a deadlock situation later.However, this works only if the system knows what requests forresources a process will be making in the future, and this isan unrealistic assumption. The text describes the bankers algorithmbut then points out that it is essentially impossible to implementbecause of this assumption.

Deadlock Prevention

The difference between deadlock avoidance and deadlock preventionis a little subtle. Deadlock avoidance refers to a strategy wherewhenever a resource is requested, it is only granted if it cannotresult in deadlock. Deadlock prevention strategies involvechanging the rules so that processes will not make requests thatcould result in deadlock.

Here is a simple example of such a strategy. Suppose every possibleresource is numbered (easy enough in theory, but often hard inpractice), and processes must make their requests in order;that is, they cannot request a resource with a number lowerthan any of the resources that they have been granted so far.Deadlock cannot occur in this situation.

As an example, consider the dining philosophers problem. Supposeeach chopstick is numbered, and philosophers always have to pickup the lower numbered chopstick before the higher numbered chopstick. Philosopher five picks up chopstick 4, philosopher 4 picks up chopstick 3,philosopher 3 picks up chopstick 2, philosopher 2 picks up chopstick 1.Philosopher 1 is hungry, and without this assumption, would pick upchopstick 5, thus causing deadlock. However, if the lower number ruleis in effect, he/she has to pick up chopstick 1 first, and it is alreadyin use, so he/she is blocked. Philosopher 5 picks up chopstick5, eats, and puts both down, allows philosopher 4 to eat.Eventually everyone gets to eat.

An alternative strategy is to require all processes to requestall of their resources at once, and either all are granted ornone are granted. Like the above strategy, this is conceptuallyeasy but often hard to implement in practice because it assumesthat a process knows what resources it will need in advance.

Livelock

There is a variant of deadlock called livelock. This is asituation in which two or more processes continuously change theirstate in response to changes in the other process(es) without doingany useful work. This is similar to deadlock in that no progress ismade but differs in that neither process is blocked or waiting foranything.

A human example of livelock would be two people who meet face-to-facein a corridor and each moves aside to let the other pass, but they endup swaying from side to side without making any progress because theyalways move the same way at the same time.

Addressing deadlock in real systems

Deadlock is a terrific theoretical problem for graduate students,but none of the solutions discussed above can be implemented ina real world, general purpose operating system. It would bedifficult to require a user program to make requests for resourcesin a certain way or in a certain order. As a result, most operatingsystems use the ostrich algorithm.

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Some specialized systems have deadlock avoidance/prevention mechanisms.For example, many database operations involve locking several records,and this can result in deadlock, so database software often hasa deadlock prevention algorithm.

The Unix file locking system lockf has a deadlock detectionmechanism built into it. Whenever a process attempts to lock afile or a record of a file, the operating system checks to see ifthat process has locked other files or records, and if it has, ituses a graph algorithm similar to the one discussed above tosee if granting that request will cause deadlock, and if it does,the request for the lock will fail, and the lockf system call willreturn and errno will be set to EDEADLK.

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Signals

Recall that an interrupt is an asynchronous event which can happenat any time. When an interrupt occurs, the processor stopsexecuting instructions in the current running process and executesan interrupt handler function in the kernel. Unix systems havea software interrupt mechanism called signals.

An example of a signal that you are probably familiar withis an interrupt signal which is sent by the user to arunning process when the user enters Control-C. The default action of this signal is to kill the process.

A signal is represented as an integer. These integersare assigned symbolic names in the header filesignal.h. The interrupt signal has thevalue 2 but you should use the symbolic name SIGINT.

Every signal has a default action. The default actionfor SIGINT is to abort the program. A program canmodify the default action for most signals or they can choose to ignore a signal.

The system call which does this has the followingfunction prototype.

void (*signal (int sig, void (*disp)(int)))(int);

This says that the function signal takes two arguments,the first, sig is a signal, and thesecond is function name. This function takesone argument, an integer and returns a pointer. The call to signal changes the signal handling function for itsfirst argument from the default to the function of itssecond argument.

Here is a simple example.

The main function calls signal to change the default action to the function SigCatcherthen enters a loop where it alternately sleeps for one second, thendisplays a message on stdout. Normally, the user couldkill this program by hitting Control-C while it wasrunning, but because the default signal action haschanged, when the user hits Control-C while this programis running, instead of the program dying, it displays themessage
Ha Ha, you can't kill me
Try it.

Notice that the signal handler function callssignal. On some Unix systems, oncea signal handler has been called, the systemresets the handler to the default unless itis reset again.

Here is a list of the predefined signals on Solaris(there are some slight differences from one Unix systemto another).Signal 11, SIGSEGV is the signal that is received whenthe program detects a segmentation fault (memory exceptionerror). The default action for this is to display the message
Segmentation Fault (core dumped)
dump the core, and terminate the program.

You can change the action for this so that it displaysa different message, but of course you cannot try tocontinue to run the program.

Signal 9, SIGKILL, is the kill signal. A program is notallowed to change the signal handler for this signal. Otherwise, it would be possible for a program to changeall of its signal handlers so that no one could killa rogue program. To send a kill signal from the shellto a particular process, enter
kill -9 ProcessNumber

Signal 14 SIGALRM sends an alarm to a process. Thedefault SIGALRM handler is to abort the program, but thiscan be changed. The system call
unsigned int alarm(unsigned int sec);
sends a SIGALRM signal to the process after secseconds. If you have changed the signal handlerfunction for this, then you can arrange for an eventto happen after a set period of time.

You can choose to ignore any signal (except SIGKILL)by using SIG_IGN as the second argument ofsignal. You can also reset the signalhandler for a particular signal to its default byusing SIG_DFL as the second argument tosignal.

Killing Zombies

Recall that if a child dies before its parent calls wait,the child becomes a zombie. In some applications, a web serverfor example, the parent forks off lots of children but doesn'tcare whether the child is dead or alive. For example, a web server mightfork a new process to handle each connection, and each childdies when the client breaks the connection. Such anapplication is at risk of producing many zombies, and zombiescan clog up the process table.

When a child dies, it sends a SIGCHLD signal to its parent. Theparent process can prevent zombies from being created bycreating a signal handler routine for SIGCHLD which calls waitwhenever it receives a SIGCHLD signal. There is no danger thatthis will cause the parent to block because it would only callwait when it knows that a child has just died.

There are several versions of wait on a Unix system. The system call waitpid has this prototype

This will function like wait in that it waits for a childto terminate, but this function allows the process to wait for a particular child by setting its first argument tothe pid that we want to wait for. However, that is notour interest here. If the first argument is set to zero,it will wait for any child to terminate, just like wait.However, the third argument can be set to WNOHANG. Thiswill cause the function to return immediately if there areno dead children. It is customary to use this functionrather than wait in the signal handler.

Here is some sample code

Pipes

Note: this topic does not real fit with the other lessons ofthe week, but you will need it for the exercise.

A second form of redirection is a pipe. A pipe is a connection betweentwo processes in which one process writes data to the pipe and theother reads from the pipe. Thus, it allows one process to pass datato another process.

The Unix system call to create a pipe is
int pipe(int fd[2])
This function takes an array of two ints (file descriptors)as an argument. It creates a pipe with fd[0] atone end and fd[1] at the other. Reading from thepipe and writing to the pipe are done with the read andwrite calls that you have seen and used before.Although bothends are opened for both reading and writing, by conventiona process writes to fd[1] and reads from fd[0].Pipes only make sense if the process calls fork after creatingthe pipe. Each process should close the end of the pipe thatit is not using. Here is a simple example in which a childsends a message to its parent through a pipe.
There is no need for the parent to wait for the child tofinish because reading from a pipe will block until there issomething in the pipe to read. If the parent runs first,it will try to execute the read statement, and will immediately block because there is nothing in the pipe. After the child writes a message to the pipe, the parent will wake up.

Pipes have a fixed size (often 4096 bytes) and if a processtries to write to a pipe which is full, the write will blockuntil a process reads some data from the pipe.

Here is a program which combines dup2 and pipe to redirect the output of the ls processto the input of the more process as would be the caseif the user typed
ls more
at the Unix command line.Here is this week's exercise