1. Motivation (Race Condition)
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1 COSC Operating Systems Design, Fall 2004, Byunggu Yu Chapter 6 Process Synchronization (textbook chapter 7) Concurrent access to shared data in the data section of a multi-thread process, in the shared memory of multiple processes, or in a shared file Although every example in this chapter is for main memory data, we can easily use the techniques for the shared data in a shared file (lower level statements = load/op/store = read/op/write). 1. Motivation (Race Condition) P1: A=A+1; P2: A=A-1; Expect A=6 Expect A=4 A=5 Figure 6.1 Concurrent access to shared data Implementation of the statement A=A+1; in machine language: Step 1.1: Register1=A Step 1.2: Register1=Register1+1 Step 1.3: A=Register1 Implementation of the statement A=A-1; in machine language: Step 2.1: Register2=A Step 2.2: Register2=Register2-1 Step 2.3: A=Register2 Race Condition: Order1: Step A=4 Order2: Step A=6 Order3: Step A=5 Inconsistency of shared data A Critical Section: A segment of Code Section that changes, updates, or writes shared data. The execution of concurrent critical sections that access the same data must be mutually exclusive. 1
2 2. Solutions to the Critical Section Problem repeat Entry Section Critical Section Exit Section Remainder Section until false Figure 6.2 Process model 2.1 Requirements Mutual Exclusion: If there are n cooperative processes or threads that update the same data, only one of them can be in its critical section at a time and no other one will be allowed to enter its critical section before current one completes its critical section. Progress: A process that is currently in its remainder section should not participate in the decision of which waiting process will enter its critical section. This decision cannot be postponed indefinitely. If Mutual Exclusion is not guaranteed, then Progress is meaningless Bounded Waiting: Once a process P has made a request to enter its critical section, the number of other process (or threads) that are allowed to access the shared data before P must be bounded. If Progress is not guaranteed, then Bounded Waiting is meaningless. 2.2 Two Process Solutions See Figure 7.2 in p Mutual exclusion (o), Progress (x), Bounded waiting (-) o P0 P1 P1 before P0: P1 cannot be granted because of P0 in the remainder section. See Figure 7.3 in p Mutual exclusion (o), Progress (x), Bounded Waiting (-) o P1 sets flag[1] right after P0 sets flag[0] and before while flag[1]~ loop: Cannot decide which one will enter its critical section 2
3 o Switching the two statements in the entry section will violate the mutual exclusion requirement. See Figure 7.4 in p Mutual exclusion (o), Progress (o), Bounded waiting (o) o With the binary variable turn, the algorithm 3 solves the problem of the algorithm Multiple Process Solutions See Figure 7.5 in p Mutual exclusion (o), Progress (o), Bounded waiting (o) o If two or more processes execute the statement number[?]=max(~ concurrently, they can have the same number. In this case, the process whose number is smaller is served first. o Since (number[k], k) of process k is unique, the second while loop in the for loop and the exit entry make the algorithm satisfy the mutual exclusion requirement. o If there is a process that is currently updating its number, the first while loop prevents other processes from comparing numbers, since the number list is not stable. o Progress1: A process that is currently in its remainder section cannot participate in the decision of which process one will enter its critical section. (number[i]=0;) o Progress2: There is always a process satisfying the while loop. Proof: if there is no process satisfying the while loop then Case 1. choosing[j] is true always, or Case 2. number[j]!= 0 and (number[j], j) < (number[i], i) is true always Case 1 is false. Case 2 is false. o Bounded waiting: (number[pid], pid) of a new process is always greater than those of the waiting processes. 2.4 Hardware Solutions Use HW to make programming task easier, especially to solve critical section problem. Critical section problem can be solved on single CPU system by disabling interrupts while a process is executing in its critical section. o However, disabling interrupts degrades responsiveness and performance, especially in multi-processor systems. Moreover, disabling interrupts frequently could affect the system s clock (e.g., clock is updated by interrupt). Instead, systems can provide atomic HW instructions. 3
4 o Load, test, modify, and store a variable in the main memory at once (i.e., atomic action) o Load two variables, swap the values and store them at once (atomic) o Note that an atomic action is performed in its entirety or is not performed at all. Test-and-Set: load, test, set, and store a value atomically o Definition: Boolean Test-and-Set (Boolean lock) { if (lock==false) lock = true; return (false); else return (true); } o Solving critical section problem with Test-and-Set instruction repeat while(test-and-set (lock) == true) do nothing; Critical Section lock=false; Remainder Section until false o However, the above algorithm does not satisfy Bounded waiting requirement. Solution Repeat waiting[i]=true; key=true; while waiting[i] and key do key=test-and-set (lock); waiting[i]=false; Critical Section j = j + 1 mod n; while (j!= i) and (!waiting[j]) do j = j + 1 mod n; if j = i then lock=false else waiting[j]=false Remainder Section Until false; Mutual exclusion: If there is a process in its critical section, lock (key) is true. So no other processes can enter the critical section before the current process sets lock or waiting value to false. 4
5 Progress: When a process gets out of the critical section, it releases lock (when there is no waiting process) by lock=false or makes the next waiting process k enter its critical section by waiting[k]=false. Therefore, no process in its remainder section can affect the decision of which one will enter its CS. Also, the decision is not postponed. Bounded waiting: The current process makes the next waiting process k enter its critical section by waiting[k]=false. Since it scans the waiting array in a cyclic fashion, the next one enters its CS within n-1 turns. 2.5 Semaphore Solutions A synchronization tool Can solve more complex synchronization problems (i.e., HW instruction solution is too complex to solve more complex problems) Definition: A semaphore S is an integer (or binary) variable that is accessed through two atomic operations wait (i.e., P) and signal (i.e., V). S is integer or binary semaphore o wait(s): while S <= 0 do nothing; (in binary, while S==0 do nothing ) S=S-1; (in binary, S=0 ) o signal(s): S=S+1; (in binary, S=1 ) Solving mutual exclusion with semaphore: Repeat wait(mutex) Critical Section signal(mutex) Remainder Section Until false Solving synchronization problem using semaphores: We want to execute the code segment 2 of process 2 after the code segment 1 of process 1. Initial value of semaphore sync is 0. Process 1 Process 2 Segment1 wait(sync) Signal(sync) Segment2 5
6 Busy waiting o All the previous solutions to critical section require busy waiting. o Busy waiting: Waiting for an event by spinning through a tight loop (e.g., while <cond.> do nothing) that polls for the event on each pass. wastes CPU time Semaphore without Busy Waiting o To overcome busy waiting, we modify the definition of semaphore, wait(), and signal() to block and resume processes to increase CPU utilization. o New Semaphore Definition: Type semaphore = record value:integer; L: List of processes; End; Each semaphore structure has an integer value and a list (queue) of processes. o wait(): S.value=S.value-1; If S.value < 0 Add calling process to S.L; Block the calling process; End If semaphore value is < 0, Pi blocks and is put on the semaphore queue. If semaphore value is >= 0, Pi enters its CS. -S.value is the number of processes in S.L (i.e., semaphore queue) o signal(): S.value=S.value+1 If S.value <= 0 Remove P from S.L; Wakeup process P; end Wakes up the next process in the semaphore queue. Change its state from block (i.e., waiting) to ready. o Semaphore list (queue) S.L can be implemented using any queuing strategy (e.g., FIFO, LIFO, Priority, ) wait() and signal() execute atomically o Uniprocessor: disable interrupt o Multiprocessor: special instruction or software CS solution. In latter case, the CS is wait() or signal() call. Software CS solution cause another spinlock & busy waiting problem. 6
7 Satisfies mutual exclusion and progress. Bounded waiting is dependent on the queuing strategy of the semaphore list. For example, LIFO queue may not satisfy bounded waiting requirement. Deadlock & Starvation o Deadlock: deadly embraced processes o e.g., Process 0 Process 1 wait(s) wait(q) wait(q) wait(s) signal(s) signal(q) signal(q) signal(s) o Starvation: occurs when a process is constantly denied access to a resource. o e.g., LIFO style S.L 3. Classical Examples In this section, we skip the bounded-buffer problem, since the example can make you confused. 3.1 Readers-Writers Problem Approach 1: no reader should be waiting unless a writer has the permission to write. Approach 2: if a writer is waiting to write, then no new reader can start reading. Solution to either variant will lead to starvation. Figures 7.14 and 7.15 in pp show a solution to the first variant. 3.2 Dining-Philosophers Problem Represent each chopstick by a semaphore. Use wait() to grab a chopstick and signal() to release it. If all philosophers are trying to eat at the same time, deadlock will occur. 7
8 Some solutions to avoid deadlock: o # of philosophers < # of chairs. o Each philosopher picks up chopsticks only if both are available (pick up both chopsticks in a single critical section) o Odd-numbered philosophers pick up the left chopstick first and then the right chopstick, whereas even-numbered philosophers pick up the right chopstick first and then the left one. High-level language constructs 4. Critical Regions & Monitors Incorrect use of semaphores can result in serious errors that are difficult to detect. Example 1: signal(mutex); Critical section wait(mutex) or omit wait() before critical section several processes may be executing in their critical section simultaneously Example 2: wait(mutex) Critical section wait(mutex) or omit signal() after critical section deadlock, no process can execute in its critical section. To deal with the above errors, some high-level language constructs, such as critical regions and monitors, have been developed. However, these are programming language issues (concurrent programming languages) and we skip the details of them in this course (i.e., skip sections 6.6 and 6.7). Usually, compilers support them. 8
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