memory barrier 和 volatile 是否足以避免数据竞争？

Question

I want to see if I am forced to use atomic integers.

I have a loop that looks similar to this:

struct loop {
  volatile int loop_variable;
  volatile int limit;
}

for (int i = loop.loop_variable ; i < loop.limit ; loop.loop_variable++) {

}

Then another thead does this:

loops.loop_variable = loops.limit;

And issues a memory barrier.

Is this multithreaded safe?

The assembly where there is a data race is between these lines:

// loop.loop_variable = loop.limit;
movl    4+loop.0(%rip), %eax                                                                                                                                                             
movl    %eax, loop.0(%rip)  

And

    // for (int i = loop.loop_variable ; i < loop.limit ; loop.loop_variable++)
    movl    loop.0(%rip), %eax                                                                                                                                                               
    movl    %eax, -4(%rbp)                                                                                                                                                                   
    jmp .L2                                                                                                                                                                                  
.L3:                                                                                                                                                                                         
    movl    loop.0(%rip), %eax                                                                                                                                                               
    addl    $1, %eax                                                                                                                                                                         
    movl    %eax, loop.0(%rip)                                                                                                                                                               
.L2:                                                                                                                                                                                         
    movl    loop.0(%rip), %eax                                                                                                                                                               
    cmpl    $99999, %eax                                                                                                                                                                     
    jle .L3                                                                                                                                                                                  
    movl    $0, %eax  

There might be a data race between

movl    loop.0(%rip), %eax                                                                                                                                                               
        addl    $1, %eax                                                                                                                                                                         
        movl    %eax, loop.0(%rip)

Since it's three instructions to increment the loop_variable. But only one to overwrite the loop variable to the limit.

Answer 1

Is this multithreaded safe?

No.

Given

loops[0].loop_variable = loops[0].limit;
< memory barrier >

in one thread, that memory barrier won't prevent int i = loop.loop_variable from reading an indeterminate value or loop.loop_variable++ producing nonsense results in another thread. Other threads can still potentially "see" the change to loops[0].loop_variable . Or parts of the change.

A memory barrier just imposes consistency afterwards - it doesn't do a thing beforehand.