when should we lock a process?

[sdf]

Hi, friends. Can you see me?
I am new here and i am reading freebsd 11.0's  kernel code since these days.
I don't know the purpose of this line of code:
=================
PROC_LOCK(p);
================
which is located at sys/kern/kern_exec.c  line:394 function:do_execve().
And let me paste its context here:
 387     /*
 388      * Lock the process and set the P_INEXEC flag to indicate that
 389      * it should be left alone until we're done here.  This is
 390      * necessary to avoid race conditions - e.g. in ptrace() -
 391      * that might allow a local user to illicitly obtain elevated
 392      * privileges.
 393      */
 394     PROC_LOCK(p);
 395     KASSERT((p->p_flag & P_INEXEC) == 0,
 396         ("%s(): process already has P_INEXEC flag", __func__));
 397     p->p_flag |= P_INEXEC;
 398     PROC_UNLOCK(p);

Could some one tell me when to lock a process? In another word, What are we doing when we are locking a process.
I can trace out its definition but i want to know more.
==================================
#define PROC_LOCK(p)    mtx_lock(&(p)->p_mtx)
==================================

Thanks!
BTW:  Is this maillist for kernel questions? If my question is not suitable here, could you  tell me which maillist  is for kernel developers?