In reply to harsh pandya:
This race is very similar to the following example
module top;
bit a,b;
initial begin
wait(a)
@(b)
$display("done1");
end
initial begin
#20
a = 1;
b = 1;
$display("done2");
end
endmodule
Because the two assignments to a and b execute in the same region, the SystemVerilog LRM section 4.7 Nondeterminsim allows the wait statement to proceed as soon as the first assignment to a occurs, or it could proceed after the display of “done2”. If it chooses the latter, the change to b has already happened and @(b) never resumes.
If I put a #0 between the two assignments, or use a non-blocking assignment to b, then @(b) is guaranteed to execute before the change to b, so “done1” will be displayed.