OpenCL compiler warning about barrier although no barrier in the code

Hi,

I am having this warning "compiler warning: limiting to 2 concurrent work_groups because threads might reach barrier out-of-order" when compiling my kernel. Here is part of my code:

__attribute__((reqd_work_group_size(10,1,1)))

__kernel void kernel1(__global int* restrict flag, __global int* restrict start, __global int* restrict end, __global volatile int* restrict producer_data){

int gid = get_global_id(0);

int sum = 0;

if(flag[gid] == 1){

for(int i = start[gid]; i< end[gid]; i++){

sum++;

producer_data[gid] = sum;

}

}

write_channel_altera(c_id, gid);

}

I know the compiler will generate this warning when there is a barrier after the for loop (Since the header of my for loop depends on global id). However, there is no barrier in my code.

Is it because OpenCL enforce a work_item order for the channel writes? I tried locks for the channel write but it didn't work. How to write the code when different work items are writing to the channel in a nondeterministic order?