[patch V4 18/36] rseq: Provide static branch for runtime debugging

Thomas Gleixner
2025-09-08T21:32:04Z

Config based debug is rarely turned on and is not available easily when
things go wrong.

Provide a static branch to allow permanent integration of debug mechanisms
along with the usual toggles in Kconfig, command line and debugfs.

Requested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
V3: Fix __setup() return value - Michael
---
Documentation/admin-guide/kernel-parameters.txt | 4 +
include/linux/rseq_entry.h | 3
init/Kconfig | 14 ++++
kernel/rseq.c | 73 ++++++++++++++++++++++--
4 files changed, 90 insertions(+), 4 deletions(-)

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6443,6 +6443,10 @@
Memory area to be used by remote processor image,
managed by CMA.

+ rseq_debug= [KNL] Enable or disable restartable sequence
+ debug mode. Defaults to CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE.
+ Format: <bool>
+
rt_group_sched= [KNL] Enable or disable SCHED_RR/FIFO group scheduling
when CONFIG_RT_GROUP_SCHED=y. Defaults to
!CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED.
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -34,6 +34,7 @@ DECLARE_PER_CPU(struct rseq_stats, rseq_
#endif /* !CONFIG_RSEQ_STATS */

#ifdef CONFIG_RSEQ
+#include <linux/jump_label.h>
#include <linux/rseq.h>

#include <linux/tracepoint-defs.h>
@@ -64,6 +65,8 @@ static inline void rseq_trace_ip_fixup(u
unsigned long offset, unsigned long abort_ip) { }
#endif /* !CONFIG_TRACEPOINT */

+DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
+
static __always_inline void rseq_note_user_irq_entry(void)
{
if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1895,10 +1895,24 @@ config RSEQ_STATS

If unsure, say N.

+config RSEQ_DEBUG_DEFAULT_ENABLE
+ default n
+ bool "Enable restartable sequences debug mode by default" if EXPERT
+ depends on RSEQ
+ help
+ This enables the static branch for debug mode of restartable
+ sequences.
+
+ This also can be controlled on the kernel command line via the
+ command line parameter "rseq_debug=0/1" and through debugfs.
+
+ If unsure, say N.
+
config DEBUG_RSEQ
default n
bool "Enable debugging of rseq() system call" if EXPERT
depends on RSEQ && DEBUG_KERNEL
+ select RSEQ_DEBUG_DEFAULT_ENABLE
help
Enable extra debugging checks for the rseq system call.

--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -95,6 +95,27 @@
RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)

+DEFINE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
+
+static inline void rseq_control_debug(bool on)
+{
+ if (on)
+ static_branch_enable(&rseq_debug_enabled);
+ else
+ static_branch_disable(&rseq_debug_enabled);
+}
+
+static int __init rseq_setup_debug(char *str)
+{
+ bool on;
+
+ if (kstrtobool(str, &on))
+ return -EINVAL;
+ rseq_control_debug(on);
+ return 1;
+}
+__setup("rseq_debug=", rseq_setup_debug);
+
#ifdef CONFIG_TRACEPOINTS
/*
* Out of line, so the actual update functions can be in a header to be
@@ -112,10 +133,11 @@ void __rseq_trace_ip_fixup(unsigned long
}
#endif /* CONFIG_TRACEPOINTS */

+#ifdef CONFIG_DEBUG_FS
#ifdef CONFIG_RSEQ_STATS
DEFINE_PER_CPU(struct rseq_stats, rseq_stats);

-static int rseq_debug_show(struct seq_file *m, void *p)
+static int rseq_stats_show(struct seq_file *m, void *p)
{
struct rseq_stats stats = { };
unsigned int cpu;
@@ -140,14 +162,56 @@ static int rseq_debug_show(struct seq_fi
return 0;
}

+static int rseq_stats_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, rseq_stats_show, inode->i_private);
+}
+
+static const struct file_operations stat_ops = {
+ .open = rseq_stats_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static int __init rseq_stats_init(struct dentry *root_dir)
+{
+ debugfs_create_file("stats", 0444, root_dir, NULL, &stat_ops);
+ return 0;
+}
+#else
+static inline void rseq_stats_init(struct dentry *root_dir) { }
+#endif /* CONFIG_RSEQ_STATS */
+
+static int rseq_debug_show(struct seq_file *m, void *p)
+{
+ bool on = static_branch_unlikely(&rseq_debug_enabled);
+
+ seq_printf(m, "%d\n", on);
+ return 0;
+}
+
+static ssize_t rseq_debug_write(struct file *file, const char __user *ubuf,
+ size_t count, loff_t *ppos)
+{
+ bool on;
+
+ if (kstrtobool_from_user(ubuf, count, &on))
+ return -EINVAL;
+
+ rseq_control_debug(on);
+ return count;
+}
+
static int rseq_debug_open(struct inode *inode, struct file *file)
{
return single_open(file, rseq_debug_show, inode->i_private);
}

-static const struct file_operations dfs_ops = {
+static const struct file_operations debug_ops = {
.open = rseq_debug_open,
.read = seq_read,
+ .write = rseq_debug_write,
.llseek = seq_lseek,
.release = single_release,
};
@@ -156,11 +220,12 @@ static int __init rseq_debugfs_init(void
{
struct dentry *root_dir = debugfs_create_dir("rseq", NULL);

- debugfs_create_file("stats", 0444, root_dir, NULL, &dfs_ops);
+ debugfs_create_file("debug", 0644, root_dir, NULL, &debug_ops);
+ rseq_stats_init(root_dir);
return 0;
}
__initcall(rseq_debugfs_init);
-#endif /* CONFIG_RSEQ_STATS */
+#endif /* CONFIG_DEBUG_FS */

#ifdef CONFIG_DEBUG_RSEQ
static struct rseq *rseq_kernel_fields(struct task_struct *t)