Thực hiện các bước iii - iv đã đề cập ở trên cho tất cả các cơ sở dữ liệu cần phải được khôi phục. Lưu ý: Sau khi các bước đã đề cập được thực thi, nó là khuyên bạn nên để đóng 'Exchange Management Console "trước khi bắt đầu khôi phục lại hoạt động. | 176 DEADLOCKS CHAP. 3 Printer Plotter Printer - ---------- Plotter Figure 3-8. Two process resource trajectories. The regions that are shaded are especially interesting. The region with lines slanting from southwest to northeast represents both processes having the printer. The mutual exclusion rule makes it impossible to enter this region. Similarly the region shaded the other way represents both processes having the plotter and is equally impossible. if the system ever enters the box bounded by Ỉ t and 2 on the sides and 5 and 6 top and bottom it will eventually deadlock when it gets to the intersection of f 2 and If . At this point A is requesting the plotter and B is requesting the printer and both are already assigned. The entire box is unsafe and must not be entered. At point Ĩ the only safe thing to do is run process A until it gets to 4. Beyond that any trajectory to u will do. The important thing to sec here is at point z 8 is requesting a resource. The system must decide whether to grant it or not. If the grant is made the system will enter an unsafe region and eventually deadlock. To avoid the deadlock B should be suspended until A has requested and released the plotter. Safe and Unsafe States The deadlock avoidance algorithms that we will study use the information of Pig. 3-6. At any instant of time there is a current state consisting of E. A c. and . A Slate is said to be safe if it is not deadlocked and there is some scheduling order in which every process can run to completion even if all of them suddenly request their maximum number of resources immediately. It is easiest to illustrate this concept by an example using one resource. In big 3-9 a we have a state in which A has 3 instances of the resource but may need as many as 9 eventually B currently has 2 and may need 4 altogether later. Similarly c also has 2 but may SEC. DEADLOCK AVOIDANCE 177 need an additional 5. A total of 10 instances of the resource exist so with 7 resources .
