11-06-2007
How many semaphores?
Hello, first of all I apologize if this thread is not in the correct section of this forum, but this one just seemed the most appropriate.
The question I have does not concern Unix specifically, it applies to virtually any OS, however it is in Unix where I learned about this problem.
So, the question is: What is the least number of semaphores needed to guarantee that n processes will run in a specified order ("chain of processes"), like this:
P1 -> P2 -> P3 -> ...... -> Pn ?
It means that P2 will not be allowed into critical section before P1 has exited it, P3 will not go before P2, and so on.
I would really appreciate if you could comment on this, since I am trying to get the correct answer for quite some time and nobody I asked could provide the answer.
Thank you.
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LEARN ABOUT LINUX
critical_enter
CRITICAL_ENTER(9) BSD Kernel Developer's Manual CRITICAL_ENTER(9)
NAME
critical_enter, critical_exit -- enter and exit a critical region
SYNOPSIS
#include <sys/param.h>
#include <sys/systm.h>
void
critical_enter(void);
void
critical_exit(void);
DESCRIPTION
These functions are used to prevent preemption in a critical region of code. All that is guaranteed is that the thread currently executing
on a CPU will not be preempted. Specifically, a thread in a critical region will not migrate to another CPU while it is in a critical
region. The current CPU may still trigger faults and exceptions during a critical section; however, these faults are usually fatal.
The critical_enter() and critical_exit() functions manage a per-thread counter to handle nested critical sections. If a thread is made
runnable that would normally preempt the current thread while the current thread is in a critical section, then the preemption will be
deferred until the current thread exits the outermost critical section.
Note that these functions are not required to provide any inter-CPU synchronization, data protection, or memory ordering guarantees and thus
should not be used to protect shared data structures.
These functions should be used with care as an infinite loop within a critical region will deadlock the CPU. Also, they should not be inter-
locked with operations on mutexes, sx locks, semaphores, or other synchronization primitives. One exception to this is that spin mutexes
include a critical section, so in certain cases critical sections may be interlocked with spin mutexes.
HISTORY
These functions were introduced in FreeBSD 5.0.
BSD October 5, 2005 BSD