The central processing units in a multiprocessor system frequently need to synchronize.
The proper multiprocessor synchronization primitives are needed.
In case, if any process on a uniprocessor system makes a system call that require to access some critical kernel table, then the kernel code can just disable interrupts before touching the table. It can do its work knowing that it will be able to finish without other process sneaking in and touching the table before finished.
Now, on a multiprocessor system, disabling interrupts affects only the central processing unit doing the disable, and other central processing unit continue to run and can still touch the critical table.
Therefore, a proper mutex protocol must be used there, and respected by all the central processing units just to guarantee that mutual exclusion works.