* [PATCH net-next 1/2] netconsole: publish the userdata payload with RCU
2026-08-03 11:30 [PATCH net-next 0/2] netconsole: replace target_list_lock by RCU on userdata hot path Breno Leitao
@ 2026-08-03 11:30 ` Breno Leitao
2026-08-04 17:09 ` Gustavo Luiz Duarte
2026-08-03 11:30 ` [PATCH net-next 2/2] selftests: netconsole: add a userdata torture test Breno Leitao
1 sibling, 1 reply; 5+ messages in thread
From: Breno Leitao @ 2026-08-03 11:30 UTC (permalink / raw)
To: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni, Shuah Khan
Cc: netdev, linux-kernel, linux-kselftest, gustavold, asantostc,
Breno Leitao, kernel-team
update_userdata() takes target_list_lock to swap nt->userdata and
nt->userdata_length, then frees the old buffer. Since commit
7eab73b18630 ("netconsole: convert to NBCON console infrastructure")
that lock is also the console's device_lock, so writing a userdata value
from configfs serialises against the printk core emitting messages.
The buffer is immutable once published, which is what RCU is for. Move
the string and its length into a single netconsole_userdata object and
publish it with rcu_replace_pointer(), freeing the old one with
kfree_rcu().
New userdata design:
0) Unify the userdata fields into a struct netcons_userdata
1) update_userdata() no longer needs target_list_lock.
2) writers stay serialised by dynamic_netconsole_mutex.
3) reading userdata needs an RCU read lock.
No functional change intended.
Signed-off-by: Breno Leitao <leitao@debian.org>
---
drivers/net/netconsole.c | 88 +++++++++++++++++++++++++++++-------------------
1 file changed, 54 insertions(+), 34 deletions(-)
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 862001d09aa84..c88c82f2136cb 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -135,13 +135,27 @@ enum target_state {
STATE_DEACTIVATED,
};
+/**
+ * struct netcons_userdata - Formatted userdata payload of a target.
+ * @rcu: Used to free the payload after a grace period.
+ * @length: Length of @data, excluding the NUL terminator.
+ * @data: Formatted " key=value\n" entries, NUL terminated.
+ *
+ * Immutable once published, so the transmit path never observes @data and
+ * @length disagreeing.
+ */
+struct netcons_userdata {
+ struct rcu_head rcu;
+ size_t length;
+ char data[];
+};
+
/**
* struct netconsole_target - Represents a configured netconsole target.
* @list: Links this target into the target_list.
* @group: Links us into the configfs subsystem hierarchy.
* @userdata_group: Links to the userdata configfs hierarchy
- * @userdata: Cached, formatted string of append
- * @userdata_length: String length of userdata.
+ * @userdata: Cached, formatted userdata payload. RCU protected.
* @sysdata: Cached, formatted string of append
* @sysdata_fields: Sysdata features enabled.
* @msgcounter: Message sent counter.
@@ -176,8 +190,7 @@ struct netconsole_target {
#ifdef CONFIG_NETCONSOLE_DYNAMIC
struct config_group group;
struct config_group userdata_group;
- char *userdata;
- size_t userdata_length;
+ struct netcons_userdata __rcu *userdata;
char sysdata[MAX_EXTRADATA_ENTRY_LEN * MAX_SYSDATA_ITEMS];
/* bit-wise with sysdata_feature bits */
@@ -1059,12 +1072,11 @@ static int calc_userdata_len(struct netconsole_target *nt)
static int update_userdata(struct netconsole_target *nt)
{
+ struct netcons_userdata *new = NULL;
+ struct netcons_userdata *old;
struct userdatum *udm_item;
struct config_item *item;
struct list_head *entry;
- char *old_buf = NULL;
- char *new_buf = NULL;
- unsigned long flags;
int offset = 0;
int len;
@@ -1076,8 +1088,8 @@ static int update_userdata(struct netconsole_target *nt)
/* Allocate new buffer */
if (len) {
- new_buf = kmalloc(len + 1, GFP_KERNEL);
- if (!new_buf)
+ new = kmalloc_flex(*new, data, len + 1);
+ if (!new)
return -ENOMEM;
}
@@ -1087,22 +1099,21 @@ static int update_userdata(struct netconsole_target *nt)
udm_item = to_userdatum(item);
/* Skip userdata with no value set */
if (udm_item->value[0]) {
- offset += scnprintf(&new_buf[offset], len + 1 - offset,
+ offset += scnprintf(&new->data[offset],
+ len + 1 - offset,
" %s=%s\n", item->ci_name,
udm_item->value);
}
}
WARN_ON_ONCE(offset != len);
+ if (new)
+ new->length = offset;
- /* Switch to new buffer and free old buffer */
- spin_lock_irqsave(&target_list_lock, flags);
- old_buf = nt->userdata;
- nt->userdata = new_buf;
- nt->userdata_length = offset;
- spin_unlock_irqrestore(&target_list_lock, flags);
-
- kfree(old_buf);
+ /* Writers are serialized by dynamic_netconsole_mutex. */
+ old = rcu_replace_pointer(nt->userdata, new,
+ lockdep_is_held(&dynamic_netconsole_mutex));
+ kfree_rcu(old, rcu);
return 0;
}
@@ -1926,14 +1937,13 @@ static void send_udp(struct netconsole_target *nt, const char *msg, int len)
static void send_msg_no_fragmentation(struct netconsole_target *nt,
const char *msg,
int msg_len,
- int release_len)
+ int release_len,
+ const struct netcons_userdata *userdata)
{
- const char *userdata = NULL;
const char *sysdata = NULL;
const char *release;
#ifdef CONFIG_NETCONSOLE_DYNAMIC
- userdata = nt->userdata;
sysdata = nt->sysdata;
#endif
@@ -1950,7 +1960,7 @@ static void send_msg_no_fragmentation(struct netconsole_target *nt,
if (userdata)
msg_len += scnprintf(&nt->buf[msg_len],
sizeof(nt->buf) - msg_len, "%s",
- userdata);
+ userdata->data);
if (sysdata)
msg_len += scnprintf(&nt->buf[msg_len],
@@ -1970,7 +1980,8 @@ static void append_release(char *buf)
static void send_fragmented_body(struct netconsole_target *nt,
const char *msgbody_ptr, int header_len,
- int msgbody_len, int sysdata_len)
+ int msgbody_len, int sysdata_len,
+ const struct netcons_userdata *userdata)
{
const char *userdata_ptr = NULL;
const char *sysdata_ptr = NULL;
@@ -1981,12 +1992,12 @@ static void send_fragmented_body(struct netconsole_target *nt,
int userdata_len = 0;
#ifdef CONFIG_NETCONSOLE_DYNAMIC
- userdata_ptr = nt->userdata;
sysdata_ptr = nt->sysdata;
- userdata_len = nt->userdata_length;
#endif
- if (WARN_ON_ONCE(!userdata_ptr && userdata_len != 0))
- return;
+ if (userdata) {
+ userdata_ptr = userdata->data;
+ userdata_len = userdata->length;
+ }
if (WARN_ON_ONCE(!sysdata_ptr && sysdata_len != 0))
return;
@@ -2063,7 +2074,8 @@ static void send_msg_fragmented(struct netconsole_target *nt,
const char *msg,
int msg_len,
int release_len,
- int sysdata_len)
+ int sysdata_len,
+ const struct netcons_userdata *userdata)
{
int header_len, msgbody_len;
const char *msgbody;
@@ -2092,7 +2104,7 @@ static void send_msg_fragmented(struct netconsole_target *nt,
* will be replaced
*/
send_fragmented_body(nt, msgbody, header_len, msgbody_len,
- sysdata_len);
+ sysdata_len, userdata);
}
/**
@@ -2107,25 +2119,33 @@ static void send_msg_fragmented(struct netconsole_target *nt,
static void send_ext_msg_udp(struct netconsole_target *nt,
struct nbcon_write_context *wctxt)
{
+ const struct netcons_userdata *userdata = NULL;
int userdata_len = 0;
int release_len = 0;
int sysdata_len = 0;
int len;
+ /* Keeps the payload picked below alive until the last send_udp(). */
+ rcu_read_lock();
+
#ifdef CONFIG_NETCONSOLE_DYNAMIC
sysdata_len = prepare_sysdata(nt, wctxt);
- userdata_len = nt->userdata_length;
+ userdata = rcu_dereference(nt->userdata);
+ if (userdata)
+ userdata_len = userdata->length;
#endif
if (nt->release)
release_len = strlen(init_utsname()->release) + 1;
len = wctxt->len + release_len + sysdata_len + userdata_len;
if (len <= MAX_PRINT_CHUNK)
- return send_msg_no_fragmentation(nt, wctxt->outbuf,
- wctxt->len, release_len);
+ send_msg_no_fragmentation(nt, wctxt->outbuf, wctxt->len,
+ release_len, userdata);
+ else
+ send_msg_fragmented(nt, wctxt->outbuf, wctxt->len, release_len,
+ sysdata_len, userdata);
- return send_msg_fragmented(nt, wctxt->outbuf, wctxt->len, release_len,
- sysdata_len);
+ rcu_read_unlock();
}
static void send_msg_udp(struct netconsole_target *nt, const char *msg,
--
2.53.0-Meta
^ permalink raw reply related [flat|nested] 5+ messages in thread* [PATCH net-next 2/2] selftests: netconsole: add a userdata torture test
2026-08-03 11:30 [PATCH net-next 0/2] netconsole: replace target_list_lock by RCU on userdata hot path Breno Leitao
2026-08-03 11:30 ` [PATCH net-next 1/2] netconsole: publish the userdata payload with RCU Breno Leitao
@ 2026-08-03 11:30 ` Breno Leitao
1 sibling, 0 replies; 5+ messages in thread
From: Breno Leitao @ 2026-08-03 11:30 UTC (permalink / raw)
To: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni, Shuah Khan
Cc: netdev, linux-kernel, linux-kselftest, gustavold, asantostc,
Breno Leitao, kernel-team
The userdata payload is rebuilt and republished on every configfs write,
including while the target is enabled and messages are being sent.
Add netcons_userdata.sh that runs random tests with userdata.
Signed-off-by: Breno Leitao <leitao@debian.org>
---
.../selftests/drivers/net/netconsole/Makefile | 1 +
.../drivers/net/netconsole/netcons_userdata.sh | 224 +++++++++++++++++++++
2 files changed, 225 insertions(+)
diff --git a/tools/testing/selftests/drivers/net/netconsole/Makefile b/tools/testing/selftests/drivers/net/netconsole/Makefile
index b56c70b7e2742..f0674c0017fc4 100644
--- a/tools/testing/selftests/drivers/net/netconsole/Makefile
+++ b/tools/testing/selftests/drivers/net/netconsole/Makefile
@@ -13,6 +13,7 @@ TEST_PROGS := \
netcons_resume.sh \
netcons_sysdata.sh \
netcons_torture.sh \
+ netcons_userdata.sh \
# end of TEST_PROGS
include ../../../lib.mk
diff --git a/tools/testing/selftests/drivers/net/netconsole/netcons_userdata.sh b/tools/testing/selftests/drivers/net/netconsole/netcons_userdata.sh
new file mode 100755
index 0000000000000..bf4c041d4d615
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/netconsole/netcons_userdata.sh
@@ -0,0 +1,224 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+
+# Exercise the netconsole userdata payload.
+#
+# The first part checks that the payload the target transmits follows what
+# configfs says: a value shows up in the next message, an update replaces the
+# previous one, clearing the value drops the entry, and so does removing the
+# key.
+#
+# The second part rewrites values, creates and deletes keys, and clears the
+# payload entirely while messages are being sent, so the transmit path keeps
+# picking up payloads that are being replaced underneath it. It runs twice,
+# once with a payload small enough to fit in a single packet and once large
+# enough to be fragmented.
+#
+# Author: Breno Leitao <leitao@debian.org>
+
+set -euo pipefail
+
+SCRIPTDIR=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
+
+source "${SCRIPTDIR}"/../lib/sh/lib_netcons.sh
+
+# Number of times each torture worker loops
+ITERATIONS=${1:-200}
+
+# Keys owned by each torture worker. Workers do not share keys, so a failing
+# configfs operation means a real problem and not a lost race.
+CHURN_KEY="churnkey"
+TRANSIENT_KEY="transientkey"
+# Number of keys used to push a message past MAX_PRINT_CHUNK
+BULK_KEYS=8
+
+USERDATA_DIR="${NETCONS_PATH}/userdata"
+# Values are capped at MAX_EXTRADATA_VALUE_LEN(200) bytes, so ${BULK_KEYS}
+# entries of this size are enough to force fragmentation
+LONG_VALUE=$(printf -- 'v%.0s' {1..190})
+
+function write_key() {
+ local KEY="${1}"
+ local VALUE="${2}"
+
+ mkdir -p "${USERDATA_DIR}/${KEY}"
+ echo "${VALUE}" > "${USERDATA_DIR}/${KEY}/value"
+}
+
+# Send a single message and capture it on the destination interface
+function send_and_capture() {
+ rm -f "${OUTPUT_FILE}"
+
+ listen_port_and_save_to "${OUTPUT_FILE}" &
+ wait_for_port "${NAMESPACE}" "${PORT}" "${IP_VERSION}"
+ echo "${MSG}: ${TARGET}" > /dev/kmsg
+ busywait "${BUSYWAIT_TIMEOUT}" test -s "${OUTPUT_FILE}" || true
+ pkill_socat
+ validate_msg "${OUTPUT_FILE}"
+}
+
+function expect_in_msg() {
+ local WANTED="${1}"
+
+ if ! grep -q -- "${WANTED}" "${OUTPUT_FILE}"; then
+ echo "FAIL: '${WANTED}' not found in ${OUTPUT_FILE}" >&2
+ cat "${OUTPUT_FILE}" >&2
+ exit "${ksft_fail}"
+ fi
+}
+
+function expect_not_in_msg() {
+ local UNWANTED="${1}"
+
+ if grep -q -- "${UNWANTED}" "${OUTPUT_FILE}"; then
+ echo "FAIL: '${UNWANTED}' found in ${OUTPUT_FILE}" >&2
+ cat "${OUTPUT_FILE}" >&2
+ exit "${ksft_fail}"
+ fi
+}
+
+# Every write publishes a new payload and frees the previous one. An empty
+# value is skipped when the payload is formatted, so this also drives the
+# target through having no payload at all.
+function churn_value() {
+ local i
+
+ for i in $(seq "${ITERATIONS}")
+ do
+ echo "value${i}" > "${USERDATA_DIR}/${CHURN_KEY}/value"
+ echo > "${USERDATA_DIR}/${CHURN_KEY}/value"
+ done
+}
+
+# Create and delete a key underneath the sender
+function churn_key() {
+ local i
+
+ for i in $(seq "${ITERATIONS}")
+ do
+ mkdir "${USERDATA_DIR}/${TRANSIENT_KEY}"
+ echo "transient${i}" > "${USERDATA_DIR}/${TRANSIENT_KEY}/value"
+ rmdir "${USERDATA_DIR}/${TRANSIENT_KEY}"
+ done
+}
+
+# Keep the transmit path busy while the payload is being replaced
+function send_messages() {
+ local i
+
+ for i in $(seq "${ITERATIONS}")
+ do
+ echo "${MSG}: ${TARGET} ${i}" > /dev/kmsg
+ done
+}
+
+# Run the workers concurrently and fail if any of them hits an error
+function run_workers() {
+ local PIDS=()
+ local WORKER
+ local PID
+
+ for WORKER in "$@"
+ do
+ "${WORKER}" &
+ PIDS+=("$!")
+ done
+
+ for PID in "${PIDS[@]}"
+ do
+ if ! wait "${PID}"
+ then
+ echo "FAIL: userdata torture worker failed" >&2
+ exit "${ksft_fail}"
+ fi
+ done
+}
+
+function create_bulk_keys() {
+ local i
+
+ for i in $(seq "${BULK_KEYS}")
+ do
+ write_key "bulk${i}" "${LONG_VALUE}"
+ done
+}
+
+function delete_bulk_keys() {
+ local i
+
+ for i in $(seq "${BULK_KEYS}")
+ do
+ rmdir "${USERDATA_DIR}/bulk${i}"
+ done
+}
+
+# ========== #
+# Start here #
+# ========== #
+
+modprobe netdevsim 2> /dev/null || true
+modprobe netconsole 2> /dev/null || true
+
+IP_VERSION="ipv4"
+# The content of kmsg will be saved to the following file
+OUTPUT_FILE="/tmp/${TARGET}"
+
+# Check for basic system dependency and exit if not found
+check_for_dependencies
+# Set current loglevel to KERN_INFO(6), and default to KERN_NOTICE(5)
+echo "6 5" > /proc/sys/kernel/printk
+# Remove the namespace, interfaces and netconsole target on exit
+trap cleanup EXIT
+# Create one namespace and two interfaces
+set_network "${IP_VERSION}"
+# Create a dynamic target for netconsole
+create_dynamic_target
+
+# ===================================================
+# TEST #1
+# A value written to configfs reaches the destination
+# ===================================================
+write_key "${USERDATA_KEY}" "first"
+send_and_capture
+expect_in_msg "${USERDATA_KEY}=first"
+
+# ===================================================
+# TEST #2
+# Updating the value replaces the previous payload
+# ===================================================
+write_key "${USERDATA_KEY}" "second"
+send_and_capture
+expect_in_msg "${USERDATA_KEY}=second"
+expect_not_in_msg "${USERDATA_KEY}=first"
+
+# ===================================================
+# TEST #3
+# Clearing the value drops the entry
+# ===================================================
+echo > "${USERDATA_DIR}/${USERDATA_KEY}/value"
+send_and_capture
+expect_not_in_msg "${USERDATA_KEY}="
+
+# ===================================================
+# TEST #4
+# Removing the key drops the entry
+# ===================================================
+write_key "${USERDATA_KEY}" "third"
+rmdir "${USERDATA_DIR}/${USERDATA_KEY}"
+send_and_capture
+expect_not_in_msg "${USERDATA_KEY}="
+rm "${OUTPUT_FILE}"
+
+# ===================================================
+# TEST #5
+# Torture the payload while messages are being sent,
+# first unfragmented and then fragmented
+# ===================================================
+write_key "${CHURN_KEY}" "${USERDATA_VALUE}"
+run_workers churn_value churn_key send_messages
+
+create_bulk_keys
+run_workers churn_value churn_key send_messages
+delete_bulk_keys
+
+exit "${ksft_pass}"
--
2.53.0-Meta
^ permalink raw reply related [flat|nested] 5+ messages in thread