multithreading - Two more more threads writing the same value to the same memory location -


i have situation several threads write same value same memory location.

can lead memory location storing corrupt value resulting concurrent writes ?

let's have object of class unique id. when used threads, these threads assign id them, 100. question : can id value other 100 after of threads write 100 memory location? in other words, have protect id mutex ?

i think multiple non-atomic writes of same value guaranteed safe (i.e. producing same result 1 write) if these 2 conditions hold:

  • a non-atomic write constructed series of atomic writes
  • several atomic writes of same value location produce same value

both of these seem natural enough expect, not sure true every possible implementation.

the example thinking of following:

suppose 2 processes write 2-byte value 1 address a. value written 2 separate atomic bytes: 1 address a, , 0 address a+1. if have 2 processes (p,q), both writing first value 1 address (say) 10, writing value 0 address 11, without mutual exclusion following possible executions:

  • p[1->10],p[0->11],q[1->10],q[0->11]
  • p[1->10],q[1->10],p[0->11],q[0->11]
  • p[1->10],q[1->10],q[0->11],p[0->11]
  • q[1->10],q[0->11],p[1->10],p[0->11]
  • q[1->10],p[1->10],q[0->11],p[0->11]
  • q[1->10],p[1->10],p[0->11],q[0->11]

either way write 1 twice location 10, , write 0 twice location 11 atomically. if 2 writes produce same result 1 write, either of above sequences produces same result.


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