[PATCH printk v4 4/6] printk: remove NMI tracking

John Ogness
2021-07-15T19:52:36Z

All NMI contexts are handled the same as the safe context: store the
message and defer printing. There is no need to have special NMI
context tracking for this. Using in_nmi() is enough.

There are several parts of the kernel that are manually calling into
the printk NMI context tracking in order to cause general printk
deferred printing:

arch/arm/kernel/smp.c
arch/powerpc/kexec/crash.c
kernel/trace/trace.c

For these users, provide a new function pair
printk_deferred_enter/exit that explicitly achieves the same
objective.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
arch/arm/kernel/smp.c | 4 ++--
arch/powerpc/kexec/crash.c | 2 +-
include/linux/hardirq.h | 2 --
include/linux/printk.h | 31 +++++++++++++++++++------------
init/Kconfig | 5 -----
kernel/printk/internal.h | 8 --------
kernel/printk/printk_safe.c | 37 +------------------------------------
kernel/trace/trace.c | 4 ++--
8 files changed, 25 insertions(+), 68 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index c7bb168b0d97..842427ff2b3c 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -667,9 +667,9 @@ static void do_handle_IPI(int ipinr)
break;

case IPI_CPU_BACKTRACE:
- printk_nmi_enter();
+ printk_deferred_enter();
nmi_cpu_backtrace(get_irq_regs());
- printk_nmi_exit();
+ printk_deferred_exit();
break;

default:
diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index 0196d0c211ac..1070378c8e35 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c
@@ -313,7 +313,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
int (*old_handler)(struct pt_regs *regs);

/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
- printk_nmi_enter();
+ printk_deferred_enter();

/*
* This function is only called after the system
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 69bc86ea382c..76878b357ffa 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -116,7 +116,6 @@ extern void rcu_nmi_exit(void);
do { \
lockdep_off(); \
arch_nmi_enter(); \
- printk_nmi_enter(); \
BUG_ON(in_nmi() == NMI_MASK); \
__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); \
} while (0)
@@ -135,7 +134,6 @@ extern void rcu_nmi_exit(void);
do { \
BUG_ON(!in_nmi()); \
__preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET); \
- printk_nmi_exit(); \
arch_nmi_exit(); \
lockdep_on(); \
} while (0)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 664612f75dac..03f7ccedaf18 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -149,18 +149,6 @@ static inline __printf(1, 2) __cold
void early_printk(const char *s, ...) { }
#endif

-#ifdef CONFIG_PRINTK_NMI
-extern void printk_nmi_enter(void);
-extern void printk_nmi_exit(void);
-extern void printk_nmi_direct_enter(void);
-extern void printk_nmi_direct_exit(void);
-#else
-static inline void printk_nmi_enter(void) { }
-static inline void printk_nmi_exit(void) { }
-static inline void printk_nmi_direct_enter(void) { }
-static inline void printk_nmi_direct_exit(void) { }
-#endif /* PRINTK_NMI */
-
struct dev_printk_info;

#ifdef CONFIG_PRINTK
@@ -180,6 +168,16 @@ int printk(const char *fmt, ...);
*/
__printf(1, 2) __cold int printk_deferred(const char *fmt, ...);

+extern void __printk_safe_enter(void);
+extern void __printk_safe_exit(void);
+/*
+ * The printk_deferred_enter/exit macros are available only as a hack for
+ * some code paths that need to defer all printk console printing. Interrupts
+ * must be disabled for the deferred duration.
+ */
+#define printk_deferred_enter __printk_safe_enter
+#define printk_deferred_exit __printk_safe_exit
+
/*
* Please don't use printk_ratelimit(), because it shares ratelimiting state
* with all other unrelated printk_ratelimit() callsites. Instead use
@@ -223,6 +221,15 @@ int printk_deferred(const char *s, ...)
{
return 0;
}
+
+static inline void printk_deferred_enter(void)
+{
+}
+
+static inline void printk_deferred_exit(void)
+{
+}
+
static inline int printk_ratelimit(void)
{
return 0;
diff --git a/init/Kconfig b/init/Kconfig
index a61c92066c2e..9c0510693543 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1506,11 +1506,6 @@ config PRINTK
very difficult to diagnose system problems, saying N here is
strongly discouraged.

-config PRINTK_NMI
- def_bool y
- depends on PRINTK
- depends on HAVE_NMI
-
config BUG
bool "BUG() support" if EXPERT
default y
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6cc35c5de890..b6d310c72fc9 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -6,12 +6,6 @@

#ifdef CONFIG_PRINTK

-#define PRINTK_SAFE_CONTEXT_MASK 0x007ffffff
-#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x008000000
-#define PRINTK_NMI_CONTEXT_MASK 0xff0000000
-
-#define PRINTK_NMI_CONTEXT_OFFSET 0x010000000
-
__printf(4, 0)
int vprintk_store(int facility, int level,
const struct dev_printk_info *dev_info,
@@ -19,8 +13,6 @@ int vprintk_store(int facility, int level,

__printf(1, 0) int vprintk_default(const char *fmt, va_list args);
__printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
-void __printk_safe_enter(void);
-void __printk_safe_exit(void);

bool printk_percpu_data_ready(void);

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 29c580dac93d..ef0f9a2044da 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -4,12 +4,9 @@
*/

#include <linux/preempt.h>
-#include <linux/spinlock.h>
-#include <linux/debug_locks.h>
#include <linux/kdb.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
-#include <linux/irq_work.h>
#include <linux/printk.h>
#include <linux/kprobes.h>

@@ -17,35 +14,6 @@

static DEFINE_PER_CPU(int, printk_context);

-#ifdef CONFIG_PRINTK_NMI
-void noinstr printk_nmi_enter(void)
-{
- this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
-}
-
-void noinstr printk_nmi_exit(void)
-{
- this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
-}
-
-/*
- * Marks a code that might produce many messages in NMI context
- * and the risk of losing them is more critical than eventual
- * reordering.
- */
-void printk_nmi_direct_enter(void)
-{
- if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
- this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
-}
-
-void printk_nmi_direct_exit(void)
-{
- this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
-}
-
-#endif /* CONFIG_PRINTK_NMI */
-
/* Can be preempted by NMI. */
void __printk_safe_enter(void)
{
@@ -70,10 +38,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
* Use the main logbuf even in NMI. But avoid calling console
* drivers that might have their own locks.
*/
- if (this_cpu_read(printk_context) &
- (PRINTK_NMI_DIRECT_CONTEXT_MASK |
- PRINTK_NMI_CONTEXT_MASK |
- PRINTK_SAFE_CONTEXT_MASK)) {
+ if (this_cpu_read(printk_context) || in_nmi()) {
int len;

len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d23a09d3eb37..b30ad20d251f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
tracing_off();

local_irq_save(flags);
- printk_nmi_direct_enter();
+ printk_deferred_enter();

/* Simulate the iterator */
trace_init_global_iter(&iter);
@@ -9729,7 +9729,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
atomic_dec(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
}
atomic_dec(&dump_running);
- printk_nmi_direct_exit();
+ printk_deferred_exit();
local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(ftrace_dump);
--
2.20.1