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Marco Realacci 2025-03-03 09:47:47 +01:00
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24
Concurrent Systems/notes/Lezione1.md
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@ -116,7 +116,7 @@ Atomic R/W registers are storage units that can be accessed through two operatio
### Peterson algorithm (for two processes) ### Peterson algorithm (for two processes)
Let's try to enforce MUTEX with just 2 processes. Let's try to enforce MUTEX with just 2 processes.
1st attempt: ##### 1st attempt:
``` ```
lock(i) := lock(i) :=
AFTER_YOU <- i AFTER_YOU <- i
@ -128,14 +128,30 @@ unlock(i) :=
``` ```
This protocol satisfies MUTEX, but suffers from deadlock (if one process never locks). This protocol satisfies MUTEX, but suffers from deadlock (if one process never locks).
2nd attempt: ##### 2nd attempt:
``` ```
Initialize FLAG[0] and FLAG[1] to down Initialize FLAG[0] and FLAG[1] to down
lock(i) := lock(i) :=
AFTER_YOU <- i FLAG[i] <- up
wait AFTER_YOU != i wait FLAG[1-i] = down
return return
unlock(i) := unlock(i) :=
FLAG[i] <- down
return
```
Still suffers from deadlock if both processes simultaneously raise their flag.
##### Correct solution:
```
Initialize FLAG[0] and FLAG[1] to down
lock(i) :=
FLAG[i] <- up
AFTER_YOU <- i
wait FLAG[1-i] = down OR AFTER_YOU != i
return
unlock(i) :=
FLAG[i] <- down
return return
``` ```