1. Geoffrey Trujillo CSE 430

2.  POSIX system calls do not handle concurrency well  Only one system call can be guaranteed to execute atomically  System cannot remain consistent when multiple system calls are made  No simple implementation can be made with locks

3.  Time-of-check-to-time-of-use  Securty risk  Race condition  Consistency of file system  Updates  Installations

4.  Easier to use than locking mechanism  Eliminate race condition  Time-of-check-to-time-of-use  Able to maintain consistency  Use of concurrent application  Software installation  Software update  Isolated from system until commit

5.  Commands  sys_xbegin, sys_xend, sys_xabort  Use  sys_xbegin(); ▪ //stuff  sys_xend();

6. if ( access (‘foo’) { symlin(‘secret’, ‘foo’); fd = open(‘foo’); write(fd); }

7. sys_xbegin(); if ( access (‘foo’) { symlink(‘secret’, ‘foo’); fd = open(‘foo’); write(fd); } sys_xend(); symlink(‘secret’, ‘foo’);

8. sys_xbegin(); //installor //update sys_xend();  Will have comlete install or update else will have original system

9.  Broken into two segments  Header  The stable part that holds the pointer to data  Data  The part the hold the private copy of the data in the system header Data

10. Header Data Private Copy Lock

11. Header Data

12. SubsystemTotal SupportPartial SupportExample Credential341Getuid, getcpu Processes133Fork, exit, exec Communication150pipe File system614Link, access, open other136Time, nanosleep Total14 SubsystemTotal SupportPartial SupportExample

13. SubSytemTotalExamples Processes33Nice, uselib Memory15Madvise Filesystem31Mount, sync Communication14Tee, select Timers8Socket, ipc Admin12Alarm Misc22Swapon, reboot 18Futex, times

14.  500,000 renames