1. TANNENBAUM SECTION 2.3 INTERPROCESS COMMUNICATION2 OPERATING SYSTEMS

2. OVERALL PROBLEM How to solve the 2-process critical section problem in software 1965 – Dekker solution. Very complex 1981 – Peterson solution. Much simpler Three variables pr_0: set means that proc0 wants to enter cs pr_1: set means that proc1 wants to enter cs turn: indicates whose turn it is

3. THE PETERSON MODEL peterson() { int turn; int turn; int pr_0 = 0; int pr_0 = 0; int pr_1 = 0; int pr_1 = 0; parbegin parbegin { proc0(); proc0(); proc1(); proc1(); } parend parend}proc0(){ pr_0 = 1; pr_0 = 1; turn = 1; turn = 1; while(pr_1 && turn); while(pr_1 && turn); { crit_sect(); crit_sect(); pr_0 = 0; pr_0 = 0; }}proc1(){ pr_1 = 1; pr_1 = 1; turn = 0; turn = 0; while(pr_0 && !turn); while(pr_0 && !turn); { crit_sect(); crit_sect(); pr_1 = 0; pr_1 = 0; }}

4. CHECK FOR MUTUAL EXCLUSION Assume both processes are in CS This implies pr_1 == 1 and pr_0 == 1 Since proc_0 is in CS either pr_1 == 0 or turn == 0 Since proc_1 is in CS either pr_0 == 0 or turn = 1 Since pr_1 and pr_0 are both 1 turn == 0 && turn == 1 This is a contradiction, so the initial assumption must be false

5. CHECK FOR REMAINING THREE CONDITIONS Speed No assumptions were made Authority proc_0 is prevented from entering only if both pr_1 and turn are set to 1. but this happens only at the mutex gate Postponement Assume that proc_0 is in a long non_crit_sect proc_1 can enter cs because proc_0 has unset pr_0

6. FOUR IPC SYSTEM CALLS shmget: creates a new region of shared memory shmat: logically attaches the newly created shared memory to the virtual address space of a process shmdt: detaches the shared memory region shmctl: deletes the shared memory region Programs using these, require: #include

7. SHMGET int shmget(int key, int size, int shmflag) Creates and opens a shared memory segment Returns an identifier that is used to provide access to the shared memory or -1 if there is an error Args: key: 0 or identifier for an existing shared memory segment size: size required in bytes shmflag: specifies how the space is to be treated (use: 0777|IPC_CREAT) Putting it together: int shmid; shmid = shmget(0, 1, 0777|IPC_CREAT) shmid, like a file descriptor, is available to the parent and all child processes

8. SHMAT int* shmat(int shmid, char *shmaddr, int shmflag); Returns the starting address of the shared memory segment or -1 if error Arguments shmid: integer returned by shmget shmaddr: if 0, unix selects the address shmflag: 0 permits both read and write int* address; address = shmat(shmid, 0, 0) address now holds the starting address of the shared memory segment address may have to be cast to an appropriate type.

9. SHMDT When a process if finished, shmdt detaches the shared memory segment int shmdt(int* address) Where address is the address returned by shmat Returns 0 or -1 indicating success or failure int value = smdt(address);

10. SHMCTL Performs the control operation identified by the second argument. We’ll use it to return a shared memory segment to the operating system. int shmctl(int shmid, int cmd, struct shmid_ds* buf) Args: shmid is shmid returned by shmget cmd: IPC_RMID buf: 0 or a data structure specified in man page. When used with IPC_RMID returns 0 for success or -1 for failure int value = shmctl(shmid, IPC_RMID, 0);

11. PROBLEMS WITH ALL FAILED SOLUTIONS Context Switch between when a lock variable was tested and set results in: Mutual Exclusion Violation Deadlock

12. WITH A LITTLE HELP FROM HARDWARE Create a single, uninterruptible hardware instruction that 1.Reads a variable (1 or 0) 2.Stores the value in a save area 3.Sets the variable

13. TSL unset_lock() { lock = 0; } enter_region: TSL register, lock//copy lock to register and set lock to 1 cmp register, #0//is lock 0? JNE enter_region//jump to label if not equal (i.e., loop) RET//return to caller leave_region: MOVE LOCK, #0//store 0 in lock RET//return to caller

14. TEST AND SET LOCK Test_Set() { int lock; unset_lock(); parbegin parbegin { proc0(); proc0(); proc1(); proc1(); } parend parend}proc0(){ enter_region(); enter_region(); crit_sect(); crit_sect(); leave_region(); leave_region();}proc1(){ enter_region() enter_region() crit_sect(); crit_sect(); leave_region() leave_region()} Trace conditions for solution to CS problem

15. TWO CORRECT SOLUTIONS TO CRITICAL SECTION PROBLEM Peterson: Software TSL: Hardware and software But 1.Both require busy-wait 2.Both are ad hoc in the sense that they are not part of programming language object