Re: [PATCH 11/11 v2] ftrace: Add recording of functions that caused recursion

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 2 Nov 2020 12:37:21 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:


> The only race that I see that can happen, is the one in the comment I
> showed. And that is after enabling the recursed functions again after
> clearing, one CPU could add a function while another CPU that just added
> that same function could be just exiting this routine, notice that a
> clearing of the array happened, and remove its function (which was the same
> as the one just happened). So we get a "zero" in the array. If this
> happens, it is likely that that function will recurse again and will be
> added later.
> 

Updated version of this function:

-- Steve


void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip)
{
	int index = 0;
	int i;
	unsigned long old;

 again:
	/* First check the last one recorded */
	if (ip == cached_function)
		return;

	i = atomic_read(&nr_records);
	/* nr_records is -1 when clearing records */
	smp_mb__after_atomic();
	if (i < 0)
		return;

	/*
	 * If there's two writers and this writer comes in second,
	 * the cmpxchg() below to update the ip will fail. Then this
	 * writer will try again. It is possible that index will now
	 * be greater than nr_records. This is because the writer
	 * that succeeded has not updated the nr_records yet.
	 * This writer could keep trying again until the other writer
	 * updates nr_records. But if the other writer takes an
	 * interrupt, and that interrupt locks up that CPU, we do
	 * not want this CPU to lock up due to the recursion protection,
	 * and have a bug report showing this CPU as the cause of
	 * locking up the computer. To not lose this record, this
	 * writer will simply use the next position to update the
	 * recursed_functions, and it will update the nr_records
	 * accordingly.
	 */
	if (index < i)
		index = i;
	if (index >= CONFIG_FTRACE_RECORD_RECURSION_SIZE)
		return;

	for (i = index - 1; i >= 0; i--) {
		if (recursed_functions[i].ip == ip) {
			cached_function = ip;
			return;
		}
	}

	cached_function = ip;

	/*
	 * We only want to add a function if it hasn't been added before.
	 * Add to the current location before incrementing the count.
	 * If it fails to add, then increment the index (save in i)
	 * and try again.
	 */
	old = cmpxchg(&recursed_functions[index].ip, 0, ip);
	if (old != 0) {
		/* Did something else already added this for us? */
		if (old == ip)
			return;
		/* Try the next location (use i for the next index) */
		index++;
		goto again;
	}

	recursed_functions[index].parent_ip = parent_ip;

	/*
	 * It's still possible that we could race with the clearing
	 *    CPU0                                    CPU1
	 *    ----                                    ----
	 *                                       ip = func
	 *  nr_records = -1;
	 *  recursed_functions[0] = 0;
	 *                                       i = -1
	 *                                       if (i < 0)
	 *  nr_records = 0;
	 *  (new recursion detected)
	 *      recursed_functions[0] = func
	 *                                            cmpxchg(recursed_functions[0],
	 *                                                    func, 0)
	 *
	 * But the worse that could happen is that we get a zero in
	 * the recursed_functions array, and it's likely that "func" will
	 * be recorded again.
	 */
	i = atomic_read(&nr_records);
	smp_mb__after_atomic();
	if (i < 0)
		cmpxchg(&recursed_functions[index].ip, ip, 0);
	else if (i <= index)
		atomic_cmpxchg(&nr_records, i, index + 1);
}