In the Compare&Swap instruction, why must the instruction execute atomically? Write about: Busy waiting - is undesirable because it’s inefficient Potential starving of a process Deadlock - low priority process and high priority process are stuck

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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In the Compare&Swap instruction, why must the instruction execute atomically?

Write about:

Busy waiting - is undesirable because it’s inefficient

Potential starving of a process

Deadlock - low priority process and high priority process are stuck

 

Compare&Swap:

// critical section based on

// compare_and_swap instruction

 

Shared variable x=0

 

while (true)

{

  while (compare_and_swap(x, 0, 1) == 1)

        // do nothing

 

   // critical section code here

   x = 0;  

   // remaining code…

}

 

// instruction logic

int compare_and_swap(int *word, int testval, int newval)

{

   int oldval;

 

   oldval = *word;

   if (oldval == testval)

       *word = newval;

 

   return oldval;

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