[PATCH v4 0/4] Cleaning printk stuff in NMI context

[PATCH v4 0/4] Cleaning printk stuff in NMI context

[Petr Mladek]

Hi Andrew,

this is just a quick respin of the previous version. It changes
the few details as you suggested. Also it fixes the build problem
on ARM as reported by Geert and Arnd.

I rather send the whole patch set because there is the renamed header.
Also the extra blank space affects two patches. I hope that it will
safe you some work. Please, let me know if you would prefer
incremental patches.


Changes against v3:

  + used size_t for "len" and "size"

  + replaced WARN() with pr_err()

  + renamed kernel/printk/printk.h -> internal.h

  + fixed build on ARM (undefined NMI_LOG_BUF_SHIFT)


Changes against v2:

  + fixed compilation problems reported by 0-day build robot

  + MN10300 and Xtensa architectures will get handled separately

  + dropped the patch that printed NMI messages directly when Oops
    in progress; it made the solution less reliable

  + made the size of the buffer configurable; use real numbers
    instead of PAGE_SIZE


Changes against v1:

  + rebased on top of 4.4-rc2; there the old implementation was
    moved to lib/nmi_backtrace.c and used also on arm; I hope that
    I got the arm side correctly; I was not able to test on arm :-(

  + defined HAVE_NMI on arm for !CPU_V7M instead of !CPU_V7;
    handle_fiq_as_nmi() is called from entry-armv.S that
    is compiled when !CPU_V7M

  + defined HAVE_NMI also on mips; it calls nmi_enter() and
    seems to have real NMIs (or am I wrong?)

  + serialized backtraces when printing directly
    (oops_in_progress)


Petr Mladek (4):
  printk/nmi: Generic solution for safe printk in NMI
  printk/nmi: Use IRQ work only when ready
  printk/nmi: Warn when some message has been lost in NMI context
  printk/nmi: Increase the size of NMI buffer and make it configurable

 arch/Kconfig                  |   7 ++
 arch/arm/Kconfig              |   2 +
 arch/arm/kernel/smp.c         |   2 +
 arch/avr32/Kconfig            |   1 +
 arch/blackfin/Kconfig         |   1 +
 arch/cris/Kconfig             |   1 +
 arch/mips/Kconfig             |   1 +
 arch/powerpc/Kconfig          |   1 +
 arch/s390/Kconfig             |   1 +
 arch/sh/Kconfig               |   1 +
 arch/sparc/Kconfig            |   1 +
 arch/tile/Kconfig             |   1 +
 arch/x86/Kconfig              |   1 +
 arch/x86/kernel/apic/hw_nmi.c |   1 -
 include/linux/hardirq.h       |   2 +
 include/linux/percpu.h        |   3 -
 include/linux/printk.h        |  12 ++-
 init/Kconfig                  |  27 +++++
 init/main.c                   |   1 +
 kernel/printk/Makefile        |   1 +
 kernel/printk/internal.h      |  55 +++++++++++
 kernel/printk/nmi.c           | 222 ++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c        |  29 +++---
 lib/nmi_backtrace.c           |  89 +----------------
 24 files changed, 356 insertions(+), 107 deletions(-)
 create mode 100644 kernel/printk/internal.h
 create mode 100644 kernel/printk/nmi.c

-- 
1.8.5.6

[PATCH v4 1/4] printk/nmi: Generic solution for safe printk in NMI

[Petr Mladek]

printk() takes some locks and could not be used a safe way in NMI
context.

The chance of a deadlock is real especially when printing
stacks from all CPUs. This particular problem has been addressed
on x86 by the commit a9edc8809328 ("x86/nmi: Perform a safe NMI stack
trace on all CPUs").

This patch reuses most of the code and makes it generic. It is
useful for all messages and architectures that support NMI.

The alternative printk_func is set when entering and is reseted when
leaving NMI context. It queues IRQ work to copy the messages into
the main ring buffer in a safe context.

__printk_nmi_flush() copies all available messages and reset
the buffer. Then we could use a simple cmpxchg operations to
get synchronized with writers. There is also used a spinlock
to get synchronized with other flushers.

We do not longer use seq_buf because it depends on external lock.
It would be hard to make all supported operations safe for
a lockless use. It would be confusing and error prone to
make only some operations safe.

The code is put into separate printk/nmi.c as suggested by
Steven Rostedt. It needs a per-CPU buffer and is compiled only
on architectures that call nmi_enter(). This is achieved by
the new HAVE_NMI Kconfig flag.

One exception is arm where the deferred printing is used for
printing backtraces even without NMI. For this purpose,
we define NEED_PRINTK_NMI Kconfig flag. The alternative
printk_func is explicitly set when IPI_CPU_BACKTRACE is
handled.

The other exceptions are MN10300 and Xtensa architectures.
We need to clean up NMI handling there first. Let's do it
separately.

The patch is heavily based on the draft from Peter Zijlstra,
see https://lkml.org/lkml/2015/6/10/327

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 arch/Kconfig                  |   7 ++
 arch/arm/Kconfig              |   2 +
 arch/arm/kernel/smp.c         |   2 +
 arch/avr32/Kconfig            |   1 +
 arch/blackfin/Kconfig         |   1 +
 arch/cris/Kconfig             |   1 +
 arch/mips/Kconfig             |   1 +
 arch/powerpc/Kconfig          |   1 +
 arch/s390/Kconfig             |   1 +
 arch/sh/Kconfig               |   1 +
 arch/sparc/Kconfig            |   1 +
 arch/tile/Kconfig             |   1 +
 arch/x86/Kconfig              |   1 +
 arch/x86/kernel/apic/hw_nmi.c |   1 -
 include/linux/hardirq.h       |   2 +
 include/linux/percpu.h        |   3 -
 include/linux/printk.h        |  12 ++-
 init/Kconfig                  |   5 +
 init/main.c                   |   1 +
 kernel/printk/Makefile        |   1 +
 kernel/printk/internal.h      |  44 +++++++++
 kernel/printk/nmi.c           | 207 ++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c        |  19 +---
 lib/nmi_backtrace.c           |  89 +-----------------
 24 files changed, 298 insertions(+), 107 deletions(-)
 create mode 100644 kernel/printk/internal.h
 create mode 100644 kernel/printk/nmi.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 4e949e58b192..7ce5101c2472 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -187,7 +187,14 @@ config HAVE_OPTPROBES
 config HAVE_KPROBES_ON_FTRACE
 	bool
 
+config HAVE_NMI
+	bool
+
+config NEED_PRINTK_NMI
+	bool
+
 config HAVE_NMI_WATCHDOG
+	depends on HAVE_NMI
 	bool
 #
 # An arch should select this if it provides all these things:
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 34e1569a11ee..96d2c275f0f7 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -63,6 +63,8 @@ config ARM
 	select HAVE_KRETPROBES if (HAVE_KPROBES)
 	select HAVE_MEMBLOCK
 	select HAVE_MOD_ARCH_SPECIFIC
+	select HAVE_NMI if (!CPU_V7M)
+	select NEED_PRINTK_NMI if (CPU_V7M)
 	select HAVE_OPROFILE if (HAVE_PERF_EVENTS)
 	select HAVE_OPTPROBES if !THUMB2_KERNEL
 	select HAVE_PERF_EVENTS
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b26361355dae..a960adb9bd7d 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -648,7 +648,9 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
 
 	case IPI_CPU_BACKTRACE:
 		irq_enter();
+		printk_nmi_enter();
 		nmi_cpu_backtrace(regs);
+		printk_nmi_exit();
 		irq_exit();
 		break;
 
diff --git a/arch/avr32/Kconfig b/arch/avr32/Kconfig
index b6878eb64884..9dc3e2b1180b 100644
--- a/arch/avr32/Kconfig
+++ b/arch/avr32/Kconfig
@@ -17,6 +17,7 @@ config AVR32
 	select GENERIC_CLOCKEVENTS
 	select HAVE_MOD_ARCH_SPECIFIC
 	select MODULES_USE_ELF_RELA
+	select HAVE_NMI
 	help
 	  AVR32 is a high-performance 32-bit RISC microprocessor core,
 	  designed for cost-sensitive embedded applications, with particular
diff --git a/arch/blackfin/Kconfig b/arch/blackfin/Kconfig
index af76634f8d98..47c0a55acafd 100644
--- a/arch/blackfin/Kconfig
+++ b/arch/blackfin/Kconfig
@@ -40,6 +40,7 @@ config BLACKFIN
 	select HAVE_MOD_ARCH_SPECIFIC
 	select MODULES_USE_ELF_RELA
 	select HAVE_DEBUG_STACKOVERFLOW
+	select HAVE_NMI
 
 config GENERIC_CSUM
 	def_bool y
diff --git a/arch/cris/Kconfig b/arch/cris/Kconfig
index e086f9e93728..62148eaf0189 100644
--- a/arch/cris/Kconfig
+++ b/arch/cris/Kconfig
@@ -69,6 +69,7 @@ config CRIS
 	select GENERIC_CLOCKEVENTS if ETRAX_ARCH_V32
 	select GENERIC_SCHED_CLOCK if ETRAX_ARCH_V32
 	select HAVE_DEBUG_BUGVERBOSE if ETRAX_ARCH_V32
+	select HAVE_NMI
 
 config HZ
 	int
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 71683a853372..35ae01fe980a 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -63,6 +63,7 @@ config MIPS
 	select HAVE_IRQ_TIME_ACCOUNTING
 	select GENERIC_TIME_VSYSCALL
 	select ARCH_CLOCKSOURCE_DATA
+	select HAVE_NMI
 
 menu "Machine selection"
 
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index db49e0d796b1..1bd8a7503a48 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -156,6 +156,7 @@ config PPC
 	select NO_BOOTMEM
 	select HAVE_GENERIC_RCU_GUP
 	select HAVE_PERF_EVENTS_NMI if PPC64
+	select HAVE_NMI if PERF_EVENTS
 	select EDAC_SUPPORT
 	select EDAC_ATOMIC_SCRUB
 	select ARCH_HAS_DMA_SET_COHERENT_MASK
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 3a55f493c7da..9a8a76829508 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -159,6 +159,7 @@ config S390
 	select TTY
 	select VIRT_CPU_ACCOUNTING
 	select VIRT_TO_BUS
+	select HAVE_NMI
 
 
 config SCHED_OMIT_FRAME_POINTER
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index d514df7e04dd..ff8093b574f1 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -44,6 +44,7 @@ config SUPERH
 	select OLD_SIGSUSPEND
 	select OLD_SIGACTION
 	select HAVE_ARCH_AUDITSYSCALL
+	select HAVE_NMI
 	help
 	  The SuperH is a RISC processor targeted for use in embedded systems
 	  and consumer electronics; it was also used in the Sega Dreamcast
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 56442d2d7bbc..e05ea973fd6a 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -80,6 +80,7 @@ config SPARC64
 	select NO_BOOTMEM
 	select HAVE_ARCH_AUDITSYSCALL
 	select ARCH_SUPPORTS_ATOMIC_RMW
+	select HAVE_NMI
 
 config ARCH_DEFCONFIG
 	string
diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
index 106c21bd7f44..050468c41dc6 100644
--- a/arch/tile/Kconfig
+++ b/arch/tile/Kconfig
@@ -29,6 +29,7 @@ config TILE
 	select HAVE_DEBUG_STACKOVERFLOW
 	select ARCH_WANT_FRAME_POINTERS
 	select HAVE_CONTEXT_TRACKING
+	select HAVE_NMI if USE_PMC
 	select EDAC_SUPPORT
 	select GENERIC_STRNCPY_FROM_USER
 	select GENERIC_STRNLEN_USER
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f22b61..e83f99194c4c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -126,6 +126,7 @@ config X86
 	select HAVE_MEMBLOCK
 	select HAVE_MEMBLOCK_NODE_MAP
 	select HAVE_MIXED_BREAKPOINTS_REGS
+	select HAVE_NMI
 	select HAVE_OPROFILE
 	select HAVE_OPTPROBES
 	select HAVE_PCSPKR_PLATFORM
diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 045e424fb368..7788ce643bf4 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -18,7 +18,6 @@
 #include <linux/nmi.h>
 #include <linux/module.h>
 #include <linux/delay.h>
-#include <linux/seq_buf.h>
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 u64 hw_nmi_get_sample_period(int watchdog_thresh)
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index dfd59d6bc6f0..a477e0766d2e 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -67,10 +67,12 @@ extern void irq_exit(void);
 		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
 		rcu_nmi_enter();				\
 		trace_hardirq_enter();				\
+		printk_nmi_enter();				\
 	} while (0)
 
 #define nmi_exit()						\
 	do {							\
+		printk_nmi_exit();				\
 		trace_hardirq_exit();				\
 		rcu_nmi_exit();					\
 		BUG_ON(!in_nmi());				\
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index caebf2a758dc..04c68b9f56f8 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -135,7 +135,4 @@ extern phys_addr_t per_cpu_ptr_to_phys(void *addr);
 	(typeof(type) __percpu *)__alloc_percpu(sizeof(type),		\
 						__alignof__(type))
 
-/* To avoid include hell, as printk can not declare this, we declare it here */
-DECLARE_PER_CPU(printk_func_t, printk_func);
-
 #endif /* __LINUX_PERCPU_H */
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9729565c25ff..7aba6b92d020 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -122,7 +122,17 @@ static inline __printf(1, 2) __cold
 void early_printk(const char *s, ...) { }
 #endif
 
-typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
+#ifdef CONFIG_PRINTK_NMI
+extern void printk_nmi_init(void);
+extern void printk_nmi_enter(void);
+extern void printk_nmi_exit(void);
+extern void printk_nmi_flush(void);
+#else
+static inline void printk_nmi_init(void) { }
+static inline void printk_nmi_enter(void) { }
+static inline void printk_nmi_exit(void) { }
+static inline void printk_nmi_flush(void) { }
+#endif /* PRINTK_NMI */
 
 #ifdef CONFIG_PRINTK
 asmlinkage __printf(5, 0)
diff --git a/init/Kconfig b/init/Kconfig
index c24b6f767bf0..c1c0b6a2d712 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1456,6 +1456,11 @@ 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 || NEED_PRINTK_NMI
+
 config BUG
 	bool "BUG() support" if EXPERT
 	default y
diff --git a/init/main.c b/init/main.c
index 9e64d7097f1a..45c1e05125dc 100644
--- a/init/main.c
+++ b/init/main.c
@@ -591,6 +591,7 @@ asmlinkage __visible void __init start_kernel(void)
 	timekeeping_init();
 	time_init();
 	sched_clock_postinit();
+	printk_nmi_init();
 	perf_event_init();
 	profile_init();
 	call_function_init();
diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
index 85405bdcf2b3..abb0042a427b 100644
--- a/kernel/printk/Makefile
+++ b/kernel/printk/Makefile
@@ -1,2 +1,3 @@
 obj-y	= printk.o
+obj-$(CONFIG_PRINTK_NMI)		+= nmi.o
 obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
new file mode 100644
index 000000000000..2de99faedfc1
--- /dev/null
+++ b/kernel/printk/internal.h
@@ -0,0 +1,44 @@
+/*
+ * internal.h - printk internal definitions
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/percpu.h>
+
+typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
+
+int __printf(1, 0) vprintk_default(const char *fmt, va_list args);
+
+#ifdef CONFIG_PRINTK_NMI
+
+/*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it temporary stores the strings into a per-CPU buffer.
+ * The alternative implementation is chosen transparently
+ * via per-CPU variable.
+ */
+DECLARE_PER_CPU(printk_func_t, printk_func);
+static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+	return this_cpu_read(printk_func)(fmt, args);
+}
+
+#else /* CONFIG_PRINTK_NMI */
+
+static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+	return vprintk_default(fmt, args);
+}
+
+#endif /* CONFIG_PRINTK_NMI */
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
new file mode 100644
index 000000000000..b5d1359402c5
--- /dev/null
+++ b/kernel/printk/nmi.c
@@ -0,0 +1,207 @@
+/*
+ * nmi.c - Safe printk in NMI context
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/preempt.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/cpumask.h>
+#include <linux/irq_work.h>
+#include <linux/printk.h>
+
+#include "internal.h"
+
+/*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it uses an alternative implementation that temporary stores
+ * the strings into a per-CPU buffer. The content of the buffer
+ * is later flushed into the main ring buffer via IRQ work.
+ *
+ * The alternative implementation is chosen transparently
+ * via @printk_func per-CPU variable.
+ *
+ * The implementation allows to flush the strings also from another CPU.
+ * There are situations when we want to make sure that all buffers
+ * were handled or when IRQs are blocked.
+ */
+DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
+
+#define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
+
+struct nmi_seq_buf {
+	atomic_t		len;	/* length of written data */
+	struct irq_work		work;	/* IRQ work that flushes the buffer */
+	unsigned char		buffer[NMI_LOG_BUF_LEN];
+};
+static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
+
+/*
+ * Safe printk() for NMI context. It uses a per-CPU buffer to
+ * store the message. NMIs are not nested, so there is always only
+ * one writer running. But the buffer might get flushed from another
+ * CPU, so we need to be careful.
+ */
+static int vprintk_nmi(const char *fmt, va_list args)
+{
+	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+	int add = 0;
+	size_t len;
+
+again:
+	len = atomic_read(&s->len);
+
+	if (len >= sizeof(s->buffer))
+		return 0;
+
+	/*
+	 * Make sure that all old data have been read before the buffer was
+	 * reseted. This is not needed when we just append data.
+	 */
+	if (!len)
+		smp_rmb();
+
+	add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
+
+	/*
+	 * Do it once again if the buffer has been flushed in the meantime.
+	 * Note that atomic_cmpxchg() is an implicit memory barrier that
+	 * makes sure that the data were written before updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, len + add) != len)
+		goto again;
+
+	/* Get flushed in a more safe context. */
+	if (add)
+		irq_work_queue(&s->work);
+
+	return add;
+}
+
+/*
+ * printk one line from the temporary buffer from @start index until
+ * and including the @end index.
+ */
+static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
+{
+	const char *buf = s->buffer + start;
+
+	printk("%.*s", (end - start) + 1, buf);
+}
+
+/*
+ * Flush data from the associated per_CPU buffer. The function
+ * can be called either via IRQ work or independently.
+ */
+static void __printk_nmi_flush(struct irq_work *work)
+{
+	static raw_spinlock_t read_lock =
+		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
+	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
+	size_t len, size;
+	int i, last_i;
+
+	/*
+	 * The lock has two functions. First, one reader has to flush all
+	 * available message to make the lockless synchronization with
+	 * writers easier. Second, we do not want to mix messages from
+	 * different CPUs. This is especially important when printing
+	 * a backtrace.
+	 */
+	raw_spin_lock(&read_lock);
+
+	i = 0;
+more:
+	len = atomic_read(&s->len);
+
+	/*
+	 * This is just a paranoid check that nobody has manipulated
+	 * the buffer an unexpected way. If we printed something then
+	 * @len must only increase.
+	 */
+	if (i && i >= len)
+		pr_err("printk_nmi_flush: internal error: i=%d >= len=%lu\n",
+		       i, len);
+
+	if (!len)
+		goto out; /* Someone else has already flushed the buffer. */
+
+	/* Make sure that data has been written up to the @len */
+	smp_rmb();
+
+	size = min_t(int, len, sizeof(s->buffer));
+	last_i = i;
+
+	/* Print line by line. */
+	for (; i < size; i++) {
+		if (s->buffer[i] == '\n') {
+			print_nmi_seq_line(s, last_i, i);
+			last_i = i + 1;
+		}
+	}
+	/* Check if there was a partial line. */
+	if (last_i < size) {
+		print_nmi_seq_line(s, last_i, size - 1);
+		pr_cont("\n");
+	}
+
+	/*
+	 * Check that nothing has got added in the meantime and truncate
+	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
+	 * barrier that makes sure that the data were copied before
+	 * updating s->len.
+	 */
+	if (atomic_cmpxchg(&s->len, len, 0) != len)
+		goto more;
+
+out:
+	raw_spin_unlock(&read_lock);
+}
+
+/**
+ * printk_nmi_flush - flush all per-cpu nmi buffers.
+ *
+ * The buffers are flushed automatically via IRQ work. This function
+ * is useful only when someone wants to be sure that all buffers have
+ * been flushed at some point.
+ */
+void printk_nmi_flush(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		__printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
+}
+
+void __init printk_nmi_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
+
+		init_irq_work(&s->work, __printk_nmi_flush);
+	}
+}
+
+void printk_nmi_enter(void)
+{
+	this_cpu_write(printk_func, vprintk_nmi);
+}
+
+void printk_nmi_exit(void)
+{
+	this_cpu_write(printk_func, vprintk_default);
+}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826f1053..480ad05ab299 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -54,6 +54,7 @@
 
 #include "console_cmdline.h"
 #include "braille.h"
+#include "internal.h"
 
 int console_printk[4] = {
 	CONSOLE_LOGLEVEL_DEFAULT,	/* console_loglevel */
@@ -1867,14 +1868,6 @@ int vprintk_default(const char *fmt, va_list args)
 }
 EXPORT_SYMBOL_GPL(vprintk_default);
 
-/*
- * This allows printk to be diverted to another function per cpu.
- * This is useful for calling printk functions from within NMI
- * without worrying about race conditions that can lock up the
- * box.
- */
-DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
-
 /**
  * printk - print a kernel message
  * @fmt: format string
@@ -1898,21 +1891,11 @@ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
  */
 asmlinkage __visible int printk(const char *fmt, ...)
 {
-	printk_func_t vprintk_func;
 	va_list args;
 	int r;
 
 	va_start(args, fmt);
-
-	/*
-	 * If a caller overrides the per_cpu printk_func, then it needs
-	 * to disable preemption when calling printk(). Otherwise
-	 * the printk_func should be set to the default. No need to
-	 * disable preemption here.
-	 */
-	vprintk_func = this_cpu_read(printk_func);
 	r = vprintk_func(fmt, args);
-
 	va_end(args);
 
 	return r;
diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
index 6019c53c669e..26caf51cc238 100644
--- a/lib/nmi_backtrace.c
+++ b/lib/nmi_backtrace.c
@@ -16,33 +16,14 @@
 #include <linux/delay.h>
 #include <linux/kprobes.h>
 #include <linux/nmi.h>
-#include <linux/seq_buf.h>
 
 #ifdef arch_trigger_all_cpu_backtrace
 /* For reliability, we're prepared to waste bits here. */
 static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
-static cpumask_t printtrace_mask;
-
-#define NMI_BUF_SIZE		4096
-
-struct nmi_seq_buf {
-	unsigned char		buffer[NMI_BUF_SIZE];
-	struct seq_buf		seq;
-};
-
-/* Safe printing in NMI context */
-static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
 
 /* "in progress" flag of arch_trigger_all_cpu_backtrace */
 static unsigned long backtrace_flag;
 
-static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
-{
-	const char *buf = s->buffer + start;
-
-	printk("%.*s", (end - start) + 1, buf);
-}
-
 /*
  * When raise() is called it will be is passed a pointer to the
  * backtrace_mask. Architectures that call nmi_cpu_backtrace()
@@ -52,8 +33,7 @@ static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
 void nmi_trigger_all_cpu_backtrace(bool include_self,
 				   void (*raise)(cpumask_t *mask))
 {
-	struct nmi_seq_buf *s;
-	int i, cpu, this_cpu = get_cpu();
+	int i, this_cpu = get_cpu();
 
 	if (test_and_set_bit(0, &backtrace_flag)) {
 		/*
@@ -68,17 +48,6 @@ void nmi_trigger_all_cpu_backtrace(bool include_self,
 	if (!include_self)
 		cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_mask));
 
-	cpumask_copy(&printtrace_mask, to_cpumask(backtrace_mask));
-
-	/*
-	 * Set up per_cpu seq_buf buffers that the NMIs running on the other
-	 * CPUs will write to.
-	 */
-	for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
-		s = &per_cpu(nmi_print_seq, cpu);
-		seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
-	}
-
 	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
 		pr_info("Sending NMI to %s CPUs:\n",
 			(include_self ? "all" : "other"));
@@ -94,73 +63,25 @@ void nmi_trigger_all_cpu_backtrace(bool include_self,
 	}
 
 	/*
-	 * Now that all the NMIs have triggered, we can dump out their
-	 * back traces safely to the console.
+	 * Force flush any remote buffers that might be stuck in IRQ context
+	 * and therefore could not run their irq_work.
 	 */
-	for_each_cpu(cpu, &printtrace_mask) {
-		int len, last_i = 0;
+	printk_nmi_flush();
 
-		s = &per_cpu(nmi_print_seq, cpu);
-		len = seq_buf_used(&s->seq);
-		if (!len)
-			continue;
-
-		/* Print line by line. */
-		for (i = 0; i < len; i++) {
-			if (s->buffer[i] == '\n') {
-				print_seq_line(s, last_i, i);
-				last_i = i + 1;
-			}
-		}
-		/* Check if there was a partial line. */
-		if (last_i < len) {
-			print_seq_line(s, last_i, len - 1);
-			pr_cont("\n");
-		}
-	}
-
-	clear_bit(0, &backtrace_flag);
-	smp_mb__after_atomic();
+	clear_bit_unlock(0, &backtrace_flag);
 	put_cpu();
 }
 
-/*
- * It is not safe to call printk() directly from NMI handlers.
- * It may be fine if the NMI detected a lock up and we have no choice
- * but to do so, but doing a NMI on all other CPUs to get a back trace
- * can be done with a sysrq-l. We don't want that to lock up, which
- * can happen if the NMI interrupts a printk in progress.
- *
- * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
- * the content into a per cpu seq_buf buffer. Then when the NMIs are
- * all done, we can safely dump the contents of the seq_buf to a printk()
- * from a non NMI context.
- */
-static int nmi_vprintk(const char *fmt, va_list args)
-{
-	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
-	unsigned int len = seq_buf_used(&s->seq);
-
-	seq_buf_vprintf(&s->seq, fmt, args);
-	return seq_buf_used(&s->seq) - len;
-}
-
 bool nmi_cpu_backtrace(struct pt_regs *regs)
 {
 	int cpu = smp_processor_id();
 
 	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
-		printk_func_t printk_func_save = this_cpu_read(printk_func);
-
-		/* Replace printk to write into the NMI seq */
-		this_cpu_write(printk_func, nmi_vprintk);
 		pr_warn("NMI backtrace for cpu %d\n", cpu);
 		if (regs)
 			show_regs(regs);
 		else
 			dump_stack();
-		this_cpu_write(printk_func, printk_func_save);
-
 		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
 		return true;
 	}
-- 
1.8.5.6

[PATCH v4 2/4] printk/nmi: Use IRQ work only when ready

[Petr Mladek]

NMIs could happen at any time. This patch makes sure that the safe
printk() in NMI will schedule IRQ work only when the related
structs are initialized.

All pending messages are flushed when the IRQ work is being
initialized.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/printk/nmi.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index b5d1359402c5..9a506bd18b9d 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -38,6 +38,7 @@
  * were handled or when IRQs are blocked.
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
+static int printk_nmi_irq_ready;
 
 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
 
@@ -84,8 +85,11 @@ again:
 		goto again;
 
 	/* Get flushed in a more safe context. */
-	if (add)
+	if (add && printk_nmi_irq_ready) {
+		/* Make sure that IRQ work is really initialized. */
+		smp_rmb();
 		irq_work_queue(&s->work);
+	}
 
 	return add;
 }
@@ -194,6 +198,13 @@ void __init printk_nmi_init(void)
 
 		init_irq_work(&s->work, __printk_nmi_flush);
 	}
+
+	/* Make sure that IRQ works are initialized before enabling. */
+	smp_wmb();
+	printk_nmi_irq_ready = 1;
+
+	/* Flush pending messages that did not have scheduled IRQ works. */
+	printk_nmi_flush();
 }
 
 void printk_nmi_enter(void)
-- 
1.8.5.6

[PATCH v4 3/4] printk/nmi: Warn when some message has been lost in NMI context

[Petr Mladek]

We could not resize the temporary buffer in NMI context. Let's warn
if a message is lost.

This is rather theoretical. printk() should not be used in NMI.
The only sensible use is when we want to print backtrace from all
CPUs. The current buffer should be enough for this purpose.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/printk/internal.h | 11 +++++++++++
 kernel/printk/nmi.c      |  5 ++++-
 kernel/printk/printk.c   | 10 ++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 2de99faedfc1..341bedccc065 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -34,6 +34,12 @@ static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	return this_cpu_read(printk_func)(fmt, args);
 }
 
+extern atomic_t nmi_message_lost;
+static inline int get_nmi_message_lost(void)
+{
+	return atomic_xchg(&nmi_message_lost, 0);
+}
+
 #else /* CONFIG_PRINTK_NMI */
 
 static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
@@ -41,4 +47,9 @@ static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	return vprintk_default(fmt, args);
 }
 
+static inline int get_nmi_message_lost(void)
+{
+	return 0;
+}
+
 #endif /* CONFIG_PRINTK_NMI */
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 9a506bd18b9d..d86f55cccf31 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -39,6 +39,7 @@
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
+atomic_t nmi_message_lost;
 
 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
 
@@ -64,8 +65,10 @@ static int vprintk_nmi(const char *fmt, va_list args)
 again:
 	len = atomic_read(&s->len);
 
-	if (len >= sizeof(s->buffer))
+	if (len >= sizeof(s->buffer)) {
+		atomic_inc(&nmi_message_lost);
 		return 0;
+	}
 
 	/*
 	 * Make sure that all old data have been read before the buffer was
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 480ad05ab299..71183a308b9e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1669,6 +1669,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	unsigned long flags;
 	int this_cpu;
 	int printed_len = 0;
+	int nmi_message_lost;
 	bool in_sched = false;
 	/* cpu currently holding logbuf_lock in this function */
 	static unsigned int logbuf_cpu = UINT_MAX;
@@ -1719,6 +1720,15 @@ asmlinkage int vprintk_emit(int facility, int level,
 					 strlen(recursion_msg));
 	}
 
+	nmi_message_lost = get_nmi_message_lost();
+	if (unlikely(nmi_message_lost)) {
+		text_len = scnprintf(textbuf, sizeof(textbuf),
+				     "BAD LUCK: lost %d message(s) from NMI context!",
+				     nmi_message_lost);
+		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
+					 NULL, 0, textbuf, text_len);
+	}
+
 	/*
 	 * The printf needs to come first; we need the syslog
 	 * prefix which might be passed-in as a parameter.
-- 
1.8.5.6

[PATCH v4 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

[Petr Mladek]

Testing has shown that the backtrace sometimes does not fit
into the 4kB temporary buffer that is used in NMI context.
The warnings are gone when I double the temporary buffer size.

This patch doubles the buffer size and makes it configurable.

Note that this problem existed even in the x86-specific
implementation that was added by the commit a9edc8809328
("x86/nmi: Perform a safe NMI stack trace on all CPUs").
Nobody noticed it because it did not print any warnings.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 init/Kconfig        | 22 ++++++++++++++++++++++
 kernel/printk/nmi.c |  3 ++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index c1c0b6a2d712..61cfd96a3c96 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -866,6 +866,28 @@ config LOG_CPU_MAX_BUF_SHIFT
 		     13 =>   8 KB for each CPU
 		     12 =>   4 KB for each CPU
 
+config NMI_LOG_BUF_SHIFT
+	int "Temporary per-CPU NMI log buffer size (12 => 4KB, 13 => 8KB)"
+	range 10 21
+	default 13
+	depends on PRINTK_NMI
+	help
+	  Select the size of a per-CPU buffer where NMI messages are temporary
+	  stored. They are copied to the main log buffer in a safe context
+	  to avoid a deadlock. The value defines the size as a power of 2.
+
+	  NMI messages are rare and limited. The largest one is when
+	  a backtrace is printed. It usually fits into 4KB. Select
+	  8KB if you want to be on the safe side.
+
+	  Examples:
+		     17 => 128 KB for each CPU
+		     16 =>  64 KB for each CPU
+		     15 =>  32 KB for each CPU
+		     14 =>  16 KB for each CPU
+		     13 =>   8 KB for each CPU
+		     12 =>   4 KB for each CPU
+
 #
 # Architectures with an unreliable sched_clock() should select this:
 #
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index d86f55cccf31..92c61e960c4a 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -41,7 +41,8 @@ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
 atomic_t nmi_message_lost;
 
-#define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
+#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -		\
+			 sizeof(atomic_t) - sizeof(struct irq_work))
 
 struct nmi_seq_buf {
 	atomic_t		len;	/* length of written data */
-- 
1.8.5.6

Re: [PATCH v4 0/4] Cleaning printk stuff in NMI context

[Petr Mladek]

On Fri 2015-12-11 14:20:48, Petr Mladek wrote:
> Hi Andrew,
> 
> this is just a quick respin of the previous version. It changes
> the few details as you suggested. Also it fixes the build problem
> on ARM as reported by Geert and Arnd.
> 
> I rather send the whole patch set because there is the renamed header.
> Also the extra blank space affects two patches. I hope that it will
> safe you some work. Please, let me know if you would prefer
> incremental patches.
> 
> 
> Changes against v3:
> 
>   + used size_t for "len" and "size"
> 
>   + replaced WARN() with pr_err()
> 
>   + renamed kernel/printk/printk.h -> internal.h
> 
>   + fixed build on ARM (undefined NMI_LOG_BUF_SHIFT)

Please, find below the diff between the two branches based on v3 and
v4 of the patch set. I hope that it might help.

diff --git a/init/Kconfig b/init/Kconfig
index efcff25a112d..61cfd96a3c96 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -870,7 +870,7 @@ config NMI_LOG_BUF_SHIFT
 	int "Temporary per-CPU NMI log buffer size (12 => 4KB, 13 => 8KB)"
 	range 10 21
 	default 13
-	depends on PRINTK && HAVE_NMI
+	depends on PRINTK_NMI
 	help
 	  Select the size of a per-CPU buffer where NMI messages are temporary
 	  stored. They are copied to the main log buffer in a safe context
diff --git a/kernel/printk/printk.h b/kernel/printk/internal.h
similarity index 97%
rename from kernel/printk/printk.h
rename to kernel/printk/internal.h
index 7f778b609492..341bedccc065 100644
--- a/kernel/printk/printk.h
+++ b/kernel/printk/internal.h
@@ -1,5 +1,5 @@
 /*
- * printk.h - printk internal definitions
+ * internal.h - printk internal definitions
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 78c07d441b4e..92c61e960c4a 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -22,7 +22,7 @@
 #include <linux/irq_work.h>
 #include <linux/printk.h>
 
-#include "printk.h"
+#include "internal.h"
 
 /*
  * printk() could not take logbuf_lock in NMI context. Instead,
@@ -60,12 +60,13 @@ static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
 static int vprintk_nmi(const char *fmt, va_list args)
 {
 	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
-	int add = 0, len;
+	int add = 0;
+	size_t len;
 
 again:
 	len = atomic_read(&s->len);
 
-	if (len >=  sizeof(s->buffer)) {
+	if (len >= sizeof(s->buffer)) {
 		atomic_inc(&nmi_message_lost);
 		return 0;
 	}
@@ -117,7 +118,8 @@ static void __printk_nmi_flush(struct irq_work *work)
 	static raw_spinlock_t read_lock =
 		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
 	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
-	int len, size, i, last_i;
+	size_t len, size;
+	int i, last_i;
 
 	/*
 	 * The lock has two functions. First, one reader has to flush all
@@ -137,7 +139,9 @@ more:
 	 * the buffer an unexpected way. If we printed something then
 	 * @len must only increase.
 	 */
-	WARN_ON(i && i >= len);
+	if (i && i >= len)
+		pr_err("printk_nmi_flush: internal error: i=%d >= len=%lu\n",
+		       i, len);
 
 	if (!len)
 		goto out; /* Someone else has already flushed the buffer. */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index a6bb0b0838f3..71183a308b9e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -54,7 +54,7 @@
 
 #include "console_cmdline.h"
 #include "braille.h"
-#include "printk.h"
+#include "internal.h"
 
 int console_printk[4] = {
 	CONSOLE_LOGLEVEL_DEFAULT,	/* console_loglevel */

Re: [PATCH v4 0/4] Cleaning printk stuff in NMI context

[Andrew Morton]

On Fri, 11 Dec 2015 14:20:48 +0100 Petr Mladek <pmladek@suse.com> wrote:

> this is just a quick respin of the previous version. It changes
> the few details as you suggested. Also it fixes the build problem
> on ARM as reported by Geert and Arnd.
> 
> I rather send the whole patch set because there is the renamed header.
> Also the extra blank space affects two patches. I hope that it will
> safe you some work. Please, let me know if you would prefer
> incremental patches.
> 
> 
> Changes against v3:
> 
>   + used size_t for "len" and "size"
> 
>   + replaced WARN() with pr_err()
> 
>   + renamed kernel/printk/printk.h -> internal.h
> 
>   + fixed build on ARM (undefined NMI_LOG_BUF_SHIFT)

So the review of the v3 patchset was ...  inconclusive.  And everyone has
gone quiet about v4.

Probably because you didn't cc the V3 discussion participants when
sending out V4.  Big mistake, sorry, I can't check everything!

But v4 is basically unaltered from v3 so can we please rev this up
again?  yay or nay?  Thanks.

Re: [PATCH v4 0/4] Cleaning printk stuff in NMI context

[Petr Mladek]

On Wed 2016-01-20 13:17:13, Andrew Morton wrote:
> On Fri, 11 Dec 2015 14:20:48 +0100 Petr Mladek <pmladek@suse.com> wrote:
> 
> > this is just a quick respin of the previous version. It changes
> > the few details as you suggested. Also it fixes the build problem
> > on ARM as reported by Geert and Arnd.
> > 
> > I rather send the whole patch set because there is the renamed header.
> > Also the extra blank space affects two patches. I hope that it will
> > safe you some work. Please, let me know if you would prefer
> > incremental patches.
> > 
> > 
> > Changes against v3:
> > 
> >   + used size_t for "len" and "size"
> > 
> >   + replaced WARN() with pr_err()
> > 
> >   + renamed kernel/printk/printk.h -> internal.h
> > 
> >   + fixed build on ARM (undefined NMI_LOG_BUF_SHIFT)
> 
> So the review of the v3 patchset was ...  inconclusive.  And everyone has
> gone quiet about v4.
>
> Probably because you didn't cc the V3 discussion participants when
> sending out V4.  Big mistake, sorry, I can't check everything!

Ah, v4 was sent too fast after v3 before others reacted. It included
rather cosmetic changes based on your (Andrew's) feedback.


> But v4 is basically unaltered from v3 so can we please rev this up
> again?  yay or nay?  Thanks.

The patch set prevents deadlocks that happen in a real life. It
increases a chance to get a valid crash dump when NMI is involved.

It does not handle well the situation when NMI is involved and
the crash dump could not get produced. But this is not handled
well even now. As Peter Zijlstra writes this a usually a real
mess when he needs to use special hacks (early_printk) anyway.

I still believe that the patch set makes sense and is acceptable
as is.

Thanks for taking care of it,
Petr

Re: [PATCH v4 0/4] Cleaning printk stuff in NMI context

[Petr Mladek]

On Thu 2016-01-21 12:30:07, Petr Mladek wrote:
> On Wed 2016-01-20 13:17:13, Andrew Morton wrote:
> > On Fri, 11 Dec 2015 14:20:48 +0100 Petr Mladek <pmladek@suse.com> wrote:
> > 
> > > this is just a quick respin of the previous version. It changes
> > > the few details as you suggested. Also it fixes the build problem
> > > on ARM as reported by Geert and Arnd.
> > > 
> > > I rather send the whole patch set because there is the renamed header.
> > > Also the extra blank space affects two patches. I hope that it will
> > > safe you some work. Please, let me know if you would prefer
> > > incremental patches.
> > > 
> > > 
> > > Changes against v3:
> > > 
> > >   + used size_t for "len" and "size"
> > > 
> > >   + replaced WARN() with pr_err()
> > > 
> > >   + renamed kernel/printk/printk.h -> internal.h
> > > 
> > >   + fixed build on ARM (undefined NMI_LOG_BUF_SHIFT)
> > 
> > So the review of the v3 patchset was ...  inconclusive.  And everyone has
> > gone quiet about v4.
> >
> > Probably because you didn't cc the V3 discussion participants when
> > sending out V4.  Big mistake, sorry, I can't check everything!
> 
> Ah, v4 was sent too fast after v3 before others reacted. It included
> rather cosmetic changes based on your (Andrew's) feedback.
> 
> 
> > But v4 is basically unaltered from v3 so can we please rev this up
> > again?  yay or nay?  Thanks.
> 
> The patch set prevents deadlocks that happen in a real life. It
> increases a chance to get a valid crash dump when NMI is involved.
> 
> It does not handle well the situation when NMI is involved and
> the crash dump could not get produced. But this is not handled
> well even now. As Peter Zijlstra writes this a usually a real
> mess when he needs to use special hacks (early_printk) anyway.
> 
> I still believe that the patch set makes sense and is acceptable
> as is.

Is anyone against getting this into the mainline for 4.6, please?

Best Regards,
Petr