Compiler warning - suggest parentheses around assignment used as truth value

When I try to compile the piece of code below, I get this warning:

warning: suggest parentheses around assignment used as truth value

Why does this happen? This is a rather common idiom, I believe. I even use something like it earlier on my code.

struct PIDList* 
getRecordForPID(struct PIDList* list, pid_t pid) {
    while(list = list->next)
        if (list->pid == pid)
            return list;

    return NULL;
}