[PATCH printk v2 0/5] printk: remove safe buffers

[PATCH printk v2 0/5] printk: remove safe buffers

[John Ogness]

Hi,

Here is v2 of a series to remove the safe buffers. v1 can be
found here [0]. The safe buffers are no longer needed because
messages can be stored directly into the log buffer from any
context.

However, the safe buffers also provided a form of recursion
protection. For that reason, explicit recursion protection is
also implemented for this series.

And finally, with the removal of the safe buffers, there is no
need for extra NMI enter/exit tracking. So this is also removed
(which includes removing config option CONFIG_PRINTK_NMI).

This series is based on the printk-rework branch of
printk/linux.git:

    commit acebb5597ff1 ("kernel/printk.c: Fixed mundane typos")

Changes since v1:

- remove the printk nmi enter/exit tracking

- remove CONFIG_PRINTK_NMI config option

- use in_nmi() to detect NMI context

- remove unused printk_safe_enter/exit macros

- after switching to the dynamic buffer, copy over NMI records
  that may have arrived during the switch window

- use local_irq_*() instead of printk_safe_*() for console
  spinning

- use separate variables rather than arrays for the per-cpu
  recursion tracking

- make @syslog_lock a mutex instead of a spin_lock

- close the wait-read window for SYSLOG_ACTION_READ

- adjust various comments and commit messages as requested

John Ogness

[0] https://lore.kernel.org/lkml/20210316233326.10778-1-john.ogness@linutronix.de

John Ogness (5):
  printk: track/limit recursion
  printk: remove safe buffers
  printk: remove NMI tracking
  printk: convert @syslog_lock to mutex
  printk: syslog: close window between wait and read

 arch/arm/kernel/smp.c          |   2 -
 arch/powerpc/kernel/traps.c    |   1 -
 arch/powerpc/kernel/watchdog.c |   5 -
 arch/powerpc/kexec/crash.c     |   3 -
 include/linux/hardirq.h        |   2 -
 include/linux/printk.h         |  22 --
 init/Kconfig                   |   5 -
 kernel/kexec_core.c            |   1 -
 kernel/panic.c                 |   3 -
 kernel/printk/internal.h       |  23 ---
 kernel/printk/printk.c         | 281 +++++++++++++++----------
 kernel/printk/printk_safe.c    | 362 +--------------------------------
 kernel/trace/trace.c           |   2 -
 lib/nmi_backtrace.c            |   6 -
 14 files changed, 171 insertions(+), 547 deletions(-)

-- 
2.20.1


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[PATCH printk v2 3/5] printk: remove NMI tracking

[John Ogness]

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.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 arch/arm/kernel/smp.c       |  2 --
 arch/powerpc/kexec/crash.c  |  3 ---
 include/linux/hardirq.h     |  2 --
 include/linux/printk.h      | 12 ------------
 init/Kconfig                |  5 -----
 kernel/printk/internal.h    |  6 ------
 kernel/printk/printk_safe.c | 37 +------------------------------------
 kernel/trace/trace.c        |  2 --
 8 files changed, 1 insertion(+), 68 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 5c48eb4fd0e5..77a720c1f402 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -671,9 +671,7 @@ static void do_handle_IPI(int ipinr)
 		break;
 
 	case IPI_CPU_BACKTRACE:
-		printk_nmi_enter();
 		nmi_cpu_backtrace(get_irq_regs());
-		printk_nmi_exit();
 		break;
 
 	default:
diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index c9a889880214..d488311efab1 100644
--- a/arch/powerpc/kexec/crash.c
+++ b/arch/powerpc/kexec/crash.c
@@ -311,9 +311,6 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
 	unsigned int i;
 	int (*old_handler)(struct pt_regs *regs);
 
-	/* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
-	printk_nmi_enter();
-
 	/*
 	 * This function is only called after the system
 	 * has panicked or is otherwise in a critical state.
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 7c9d6a2d7e90..0926e9ca4d85 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -115,7 +115,6 @@ 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)
@@ -134,7 +133,6 @@ 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 2476796c1150..77f66625706e 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
diff --git a/init/Kconfig b/init/Kconfig
index 096e1af5c586..ea58c0d30a97 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1482,11 +1482,6 @@ 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..ff890ae3ee6a 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,
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 4b5df5c27334..4da1ab3332d6 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()) {
 		unsigned long flags;
 		int len;
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b5815a022ecc..83a3663d1dc0 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9344,7 +9344,6 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 	tracing_off();
 
 	local_irq_save(flags);
-	printk_nmi_direct_enter();
 
 	/* Simulate the iterator */
 	trace_init_global_iter(&iter);
@@ -9424,7 +9423,6 @@ 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();
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(ftrace_dump);
-- 
2.20.1


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Re: [PATCH printk v2 3/5] printk: remove NMI tracking

[Petr Mladek]

On Tue 2021-03-30 17:35:10, John Ogness wrote:
> 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.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

This is another great win!

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

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