[PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock

John Ogness
2021-08-03T13:13:10Z

kgdb makes use of its own cpulock (@dbg_master_lock, @kgdb_active)
during cpu roundup. This will conflict with the printk cpulock.
Therefore, a CPU must ensure that it is not holding the printk
cpulock when calling kgdb_cpu_enter(). If it is, it must allow its
printk context to complete first.

A new helper function kgdb_roundup_delay() is introduced for kgdb
to determine if it is holding the printk cpulock. If so, a flag is
set so that when the printk cpulock is released, kgdb will be
re-triggered for that CPU.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
arch/powerpc/include/asm/smp.h | 1 +
arch/powerpc/kernel/kgdb.c | 10 +++++++-
arch/powerpc/kernel/smp.c | 5 ++++
arch/x86/kernel/kgdb.c | 9 ++++---
include/linux/kgdb.h | 3 +++
include/linux/printk.h | 8 ++++++
kernel/debug/debug_core.c | 45 ++++++++++++++++++++--------------
kernel/printk/printk.c | 12 +++++++++
8 files changed, 70 insertions(+), 23 deletions(-)

diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index 03b3d010cbab..eec452e647b3 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/arch/powerpc/include/asm/smp.h
@@ -58,6 +58,7 @@ struct smp_ops_t {

extern int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64 delay_us);
extern int smp_send_safe_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64 delay_us);
+extern void smp_send_debugger_break_cpu(unsigned int cpu);
extern void smp_send_debugger_break(void);
extern void start_secondary_resume(void);
extern void smp_generic_give_timebase(void);
diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
index bdee7262c080..d57d37497862 100644
--- a/arch/powerpc/kernel/kgdb.c
+++ b/arch/powerpc/kernel/kgdb.c
@@ -120,11 +120,19 @@ int kgdb_skipexception(int exception, struct pt_regs *regs)

static int kgdb_debugger_ipi(struct pt_regs *regs)
{
- kgdb_nmicallback(raw_smp_processor_id(), regs);
+ int cpu = raw_smp_processor_id();
+
+ if (!kgdb_roundup_delay(cpu))
+ kgdb_nmicallback(cpu, regs);
return 0;
}

#ifdef CONFIG_SMP
+void kgdb_roundup_cpu(unsigned int cpu)
+{
+ smp_send_debugger_break_cpu(cpu);
+}
+
void kgdb_roundup_cpus(void)
{
smp_send_debugger_break();
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 447b78a87c8f..816d7f09bbf9 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -582,6 +582,11 @@ static void debugger_ipi_callback(struct pt_regs *regs)
debugger_ipi(regs);
}

+void smp_send_debugger_break_cpu(unsigned int cpu)
+{
+ smp_send_nmi_ipi(cpu, debugger_ipi_callback, 1000000);
+}
+
void smp_send_debugger_break(void)
{
smp_send_nmi_ipi(NMI_IPI_ALL_OTHERS, debugger_ipi_callback, 1000000);
diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 3a43a2dee658..37bd37cdf2b6 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -502,9 +502,12 @@ static int kgdb_nmi_handler(unsigned int cmd, struct pt_regs *regs)
if (atomic_read(&kgdb_active) != -1) {
/* KGDB CPU roundup */
cpu = raw_smp_processor_id();
- kgdb_nmicallback(cpu, regs);
- set_bit(cpu, was_in_debug_nmi);
- touch_nmi_watchdog();
+
+ if (!kgdb_roundup_delay(cpu)) {
+ kgdb_nmicallback(cpu, regs);
+ set_bit(cpu, was_in_debug_nmi);
+ touch_nmi_watchdog();
+ }

return NMI_HANDLED;
}
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index 258cdde8d356..9bca0d98db5a 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -212,6 +212,8 @@ extern void kgdb_call_nmi_hook(void *ignored);
*/
extern void kgdb_roundup_cpus(void);

+extern void kgdb_roundup_cpu(unsigned int cpu);
+
/**
* kgdb_arch_set_pc - Generic call back to the program counter
* @regs: Current &struct pt_regs.
@@ -365,5 +367,6 @@ extern void kgdb_free_init_mem(void);
#define dbg_late_init()
static inline void kgdb_panic(const char *msg) {}
static inline void kgdb_free_init_mem(void) { }
+static inline void kgdb_roundup_cpu(unsigned int cpu) {}
#endif /* ! CONFIG_KGDB */
#endif /* _KGDB_H_ */
diff --git a/include/linux/printk.h b/include/linux/printk.h
index ac738d1d9934..974ea2c99749 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -280,10 +280,18 @@ static inline void dump_stack(void)
extern int __printk_cpu_trylock(void);
extern void __printk_wait_on_cpu_lock(void);
extern void __printk_cpu_unlock(void);
+extern bool kgdb_roundup_delay(unsigned int cpu);
+
#else
+
#define __printk_cpu_trylock() 1
#define __printk_wait_on_cpu_lock()
#define __printk_cpu_unlock()
+
+static inline bool kgdb_roundup_delay(unsigned int cpu)
+{
+ return false;
+}
#endif /* CONFIG_SMP */

/**
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index b4aa6bb6b2bd..9117ca86b81c 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -241,35 +241,42 @@ NOKPROBE_SYMBOL(kgdb_call_nmi_hook);
static DEFINE_PER_CPU(call_single_data_t, kgdb_roundup_csd) =
CSD_INIT(kgdb_call_nmi_hook, NULL);

-void __weak kgdb_roundup_cpus(void)
+void __weak kgdb_roundup_cpu(unsigned int cpu)
{
call_single_data_t *csd;
+ int ret;
+
+ csd = &per_cpu(kgdb_roundup_csd, cpu);
+
+ /*
+ * If it didn't round up last time, don't try again
+ * since smp_call_function_single_async() will block.
+ *
+ * If rounding_up is false then we know that the
+ * previous call must have at least started and that
+ * means smp_call_function_single_async() won't block.
+ */
+ if (kgdb_info[cpu].rounding_up)
+ return;
+ kgdb_info[cpu].rounding_up = true;
+
+ ret = smp_call_function_single_async(cpu, csd);
+ if (ret)
+ kgdb_info[cpu].rounding_up = false;
+}
+NOKPROBE_SYMBOL(kgdb_roundup_cpu);
+
+void __weak kgdb_roundup_cpus(void)
+{
int this_cpu = raw_smp_processor_id();
int cpu;
- int ret;

for_each_online_cpu(cpu) {
/* No need to roundup ourselves */
if (cpu == this_cpu)
continue;

- csd = &per_cpu(kgdb_roundup_csd, cpu);
-
- /*
- * If it didn't round up last time, don't try again
- * since smp_call_function_single_async() will block.
- *
- * If rounding_up is false then we know that the
- * previous call must have at least started and that
- * means smp_call_function_single_async() won't block.
- */
- if (kgdb_info[cpu].rounding_up)
- continue;
- kgdb_info[cpu].rounding_up = true;
-
- ret = smp_call_function_single_async(cpu, csd);
- if (ret)
- kgdb_info[cpu].rounding_up = false;
+ kgdb_roundup_cpu(cpu);
}
}
NOKPROBE_SYMBOL(kgdb_roundup_cpus);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3d0c933937b4..1b546e117f10 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -44,6 +44,7 @@
#include <linux/irq_work.h>
#include <linux/ctype.h>
#include <linux/uio.h>
+#include <linux/kgdb.h>
#include <linux/sched/clock.h>
#include <linux/sched/debug.h>
#include <linux/sched/task_stack.h>
@@ -214,6 +215,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
#ifdef CONFIG_SMP
static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
static atomic_t printk_cpulock_nested = ATOMIC_INIT(0);
+static unsigned int kgdb_cpu = -1;

/**
* __printk_wait_on_cpu_lock() - Busy wait until the printk cpu-reentrant
@@ -325,6 +327,16 @@ void __printk_cpu_unlock(void)
-1); /* LMM(__printk_cpu_unlock:B) */
}
EXPORT_SYMBOL(__printk_cpu_unlock);
+
+bool kgdb_roundup_delay(unsigned int cpu)
+{
+ if (cpu != atomic_read(&printk_cpulock_owner))
+ return false;
+
+ kgdb_cpu = cpu;
+ return true;
+}
+EXPORT_SYMBOL(kgdb_roundup_delay);
#endif /* CONFIG_SMP */

/* Number of registered extended console drivers. */
--
2.20.1