CGS 3763 Operating Systems Concepts Spring 2013 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 11:30 - 12:30 AM

Lecture 28 – Monday, March 25, 2013 Last time: Today Next time Deadlock detection Wait-for-graphs Semaphores Today Monitors Atomicity Hardware support for atomicity Coordination with a bounded buffer Next time Storage models Reading assignments Chapters 6 and 7 of the textbook Lecture 28

Monitors Semaphores can be used incorrectly multiple threads may be allowed to enter the critical section guarded by the semaphore may cause deadlocks Threads may access the shared data directly without checking the semaphore. Solution  encapsulate shared data with access methods to operate on them. Monitors  an abstract data type that allows access to shared data with specific methods that guarantee mutual exclusion Lecture 28

Lecture 28

Atomic operations Concurrency control requires atomic operations. Atomic operation operation consisting of multiple steps that have to executed without any interruption, all steps must be executed or none of them. All-or-Nothing atomicity  A sequence of steps is an all-or-nothing action if, from the point of view of its invoker, the sequence always either completes, or aborts in such a way that it appears that the sequence had never been undertaken in the first place. That is, it backs out. Before-or-After atomicity  actions whose effect from the point of view of their invokers is the same as if the actions occurred either completely before or completely after one another. Atomicity requires hardware support, special instructions. Lecture 28

Lecture 28

Hardware support for atomicity It is not possible to implement atomic operations without some hardware support. All processors include in the instruction set an instruction for implementing atomic operations: Compare and Swap or Test and Test or Read and Set Memory Lecture 28

Compare-and-swap instruction Compare-and-swap instruction (CMPSWP)   atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value. This is done as a single atomic operation.  We can use CMPSWP to implement a semaphore as follows: read the value in the memory location; add one to the value use compare-and-swap to write the incremented value back retry if the value read in by the compare-and-swap did not match the value we originally read Since the compare-and-swap occurs (or appears to occur) instantaneously, if another thread updates the location while we are in-progress, the compare-and-swap is guaranteed to fail. Lecture 28

Test and Set Test-and-Set  instruction used to write to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. If multiple threads may access the same memory, and if a process is currently performing a test-and-set, no other thread may begin another test-and-set until the first one is done. A lock can be implemented using the test-and-set instruction function Lock(boolean *lock) { while (test_and_set(lock) == 1); } Lecture 28

Read and Set Memory- RSM instruction Lecture 28

What if the locking is not atomic? Lecture 28

Thread coordination with a bounded buffer Producer-consumer problem  two threads cooperate – the producer is writing in a buffer and the consumer is reading from the buffer. Basic assumptions: We have only two threads Threads proceed concurrently at independent speeds/rates Bounded buffer – only N buffer cells Messages are of fixed size and occupy only one buffer cell. Lecture 28

Lecture 28

Implicit assumptions for the correctness of the implementation One sending and one receiving thread. Only one thread updates each shared variable. Sender and receiver threads run on different processors to allow spin locks in and out are implemented as integers large enough so that they do not overflow (e.g., 64 bit integers) The shared memory used for the buffer provides read/write coherence The memory provides before-or-after atomicity for the shared variables in and out The result of executing a statement becomes visible to all threads in program order. No compiler optimization supported Lecture 28

Race condition affecting the pointers; both threads A and B increment the pointer “in” (the pointer where the data is written. Lecture 28

Lecture 28

Storage models Cell storage Journal storage Lecture 28

Desirable properties of cell storage Lecture 28

Asynchronous events and signals Signals, or software interrupts, were originally introduced in Unix to notify a process about the occurrence of a particular event in the system. Signals are analogous to hardware I/O interrupts: When a signal arrives, control will abruptly switch to the signal handler. When the handler is finished and returns, control goes back to where it came from After receiving a signal, the receiver reacts to it in a well-defined manner. That is, a process can tell the system (OS) what they want to do when signal arrives: Ignore it. Catch it and deliver it. In this case, it must specify (register) the signal handling procedure. This procedure resides in the user space. The kernel will make a call to this procedure during the signal handling and control returns to kernel after it is done. Kill the process (default for most signals). Examples: Event - child exit, signal - to parent. Control signal from keyboard. Lecture 28

Solutions to thread coordination problems must satisfy a set of conditions Safety: The required condition will never be violated. Liveness: The system should eventually progress irrespective of contention. Freedom From Starvation: No process should be denied progress for ever. That is, every process should make progress in a finite time. Bounded Wait: Every process is assured of not more than a fixed number of overtakes by other processes in the system before it makes progress. Fairness: dependent on the scheduling algorithm • FIFO: No process will ever overtake another process. • LRU: The process which received the service least recently gets the service next. For example for the mutual exclusion problem the solution should guarantee that: Safety  the mutual exclusion property is never violated Liveness  a thread will access the shared resource in a finite time Freedom for starvation  a thread will access the shared resource in a finite time Bounded wait  a thread will access the shared resource at least after a fixed number of accesses by other threads. Lecture 28

Thread coordination problems Dining philosophers Critical section Lecture 28

A solution to critical section problem Applies only to two threads Ti and Tj with i,j ={0,1} which share integer turn  if turn=i then it is the turn of Ti to enter the critical section boolean flag[2]  if flag[i]= TRUE then Ti is ready to enter the critical section To enter the critical section thread Ti sets flag[i]= TRUE sets turn=j If both threads want to enter then turn will end up with a value of either i or j and the corresponding thread will enter the critical section. Ti enters the critical section only if either flag[j]= FALSE or turn=i The solution is correct Mutual exclusion is guaranteed The liveliness is ensured The bounded-waiting is met But this solution may not work as load and store instructions can be interrupted on modern computer architectures Lecture 28

Lecture 28

Signals state and implementation A signal has the following states: Signal send - A process can send signal to one of its group member process (parent, sibling, children, and further descendants). Signal delivered - Signal bit is set. Pending signal - delivered but not yet received (action has not been taken). Signal lost - either ignored or overwritten. Implementation: Each process has a kernel space (created by default) called signal descriptor having bits for each signal. Setting a bit is delivering the signal, and resetting the bit is to indicate that the signal is received. A signal could be blocked/ignored. This requires an additional bit for each signal. Most signals are system controlled signals. Lecture 28

Locks; Before-or-After actions Locks shared variables which acts as a flag to coordinate access to a shared data. Manipulated with two primitives ACQUIRE RELEASE Support implementation of Before-or-After actions; only one thread can acquire the lock, the others have to wait. All threads must obey the convention regarding the locks. The two operations ACQUIRE and RELEASE must be atomic. Hardware support for implementation of locks RSM – Read and Set Memory CMP –Compare and Swap RSM (mem) If mem=LOCKED then RSM returns r=LOCKED and sets mem=LOCKED If mem=UNLOCKED the RSM returns r=LOCKED and sets mem=LOCKED Lecture 28