HCI UART simultaneous advertising and scanning MPU fault

Question

I'm getting an occasional MPU fault on the nRF52840 on the Thingy91 when starting scanning:

[30:48:55.021,636] [1;31m<err> os: ***** MPU FAULT *****[0m
[30:48:55.021,636] [1;31m<err> os:   Data Access Violation[0m
[30:48:55.021,667] [1;31m<err> os:   MMFAR Address: 0xc[0m
[30:48:55.021,667] [1;31m<err> os: r0/a1:  0x00000008  r1/a2:  0x00000007  r2/a3:  0x00004e0a[0m
[30:48:55.021,667] [1;31m<err> os: r3/a4:  0x00000008 r12/ip:  0x00000000 r14/lr:  0x00002ae3[0m
[30:48:55.021,667] [1;31m<err> os:  xpsr:  0x21000000[0m
[30:48:55.021,667] [1;31m<err> os: Faulting instruction address (r15/pc): 0x0001c8d6[0m
[30:48:55.021,697] [1;31m<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0[0m

Running arm-none-eabi-addr2line leads me to

buf->len -= len;

in this call below:

void *net_buf_simple_pull(struct net_buf_simple *buf, size_t len)
{
	NET_BUF_SIMPLE_DBG("buf %p len %zu", buf, len);

	__ASSERT_NO_MSG(buf->len >= len);

	buf->len -= len;
	return buf->data += len;
}

Which is the cal to net_buf_pull in tx_isr of the hci_lpuart code.

It's only occasionally (happens > 24hours of constant use). The biggest change in bluetooth functionally was the addition of periodic advertisements.

Here are the scan settings:

    struct bt_le_scan_param scan_param = {
        .type = BT_LE_SCAN_TYPE_ACTIVE,
        .options = BT_LE_SCAN_OPT_NONE,
        .interval = BT_GAP_SCAN_FAST_INTERVAL,
        .window = BT_GAP_SCAN_FAST_WINDOW,
    };

    struct bt_scan_init_param scan_init = {
        .connect_if_match = 0,
        .scan_param = &scan_param,
        .conn_param = NULL,
    };

Here are the advertising settings:

/* Advertising parameters */
struct bt_le_adv_param param_conn = BT_LE_ADV_PARAM_INIT(BT_LE_ADV_OPT_CONNECTABLE |
                                                             BT_LE_ADV_OPT_USE_NAME,
                                                         BT_GAP_ADV_SLOW_INT_MIN,
                                                         BT_GAP_ADV_SLOW_INT_MAX, NULL);

We're running NCS 1.9.1. I also scoured the issues on Zephyr's Github but didn't find any pertinent issues that match this one.

Any suggestions would be greatly appreciated.

Thanks!

HCI UART simultaneous advertising and scanning MPU fault

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