synchronization

                             synchronization is mechanism refers to the orderly execution of processes so that it can ensure that only one process executing in critical section(mutual exclusion).it is implemented in kernel mode.
note: critical section:
Here critical section refers to the memory shared by two or more processes.
ex:
Consider two processes increment and decrement. These two processes perform operations on common variable counter which is stored in memory.
We can divide the counter++ process in to few atomic operations.
                                 register1=counter             //value of counter loaded to register from memory
                                 register1=register1+1        //value is incremented by one
                                 counter=register1             //again updated value is stored in memory
 same way counter-- can be divided as
                                 register2=counter
                                 register2=register2-1
                                 counter=register2
  Above three atomic in two processes must execute in order to have good result.
  Let counter=9
                If we execute increment and decrement processes sequentially, the result is same 9.
  But if we allow to access these two processes to manipulate counter  in which the atomic operations presented  previously interleaved in some arbitrary order.one such interleaving is
      T0:    increment       execute      register1=counter              {register1=9}
      T1:    increment       execute      register1=counter+1          {register1=10}
      T2:    decrement      execute      register2=counter              {register2=9}
      T3:    decrement      execute      register2=counter-1           {register1=8}
      T4:    increment       execute      counter=register1              {register1=10}
      T5:    decrement      execute      counter=register2              {register1=8}
     We come across three possible solutions after executing the sequence.
     1) Above sequence will result  8.
     2) If we reverse the order of execution of T4 and T5, the result is 10.
     3) If two processes executer sequentially, the result is 9.
      Above three possibility results in race condition.

       To avoid race condition ,we should follow some algorithms that will result in sequential execution of each process.