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printk/nmi: generic solution for safe printk in NMI

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 a9edc88 ("x86/nmi: Perform a safe NMI stack trace on all
CPUs").

The patchset brings two big advantages.  First, it makes the NMI
backtraces safe on all architectures for free.  Second, it makes all NMI
messages almost safe on all architectures (the temporary buffer is
limited.  We still should keep the number of messages in NMI context at
minimum).

Note that there already are several messages printed in NMI context:
WARN_ON(in_nmi()), BUG_ON(in_nmi()), anything being printed out from MCE
handlers.  These are not easy to avoid.

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.

The 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

[arnd@arndb.de: printk-nmi: use %zu format string for size_t]
[akpm@linux-foundation.org: min_t->min - all types are size_t here]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Jan Kara <jack@suse.cz>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>	[arm part]
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>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Petr Mladek authored and torvalds committed May 21, 2016
1 parent 2eeed7e commit 42a0bb3f71383b457a7db362f1c69e7afb96732b
@@ -187,7 +187,11 @@ config HAVE_OPTPROBES
config HAVE_KPROBES_ON_FTRACE
bool

config HAVE_NMI
bool

config HAVE_NMI_WATCHDOG
depends on HAVE_NMI
bool
#
# An arch should select this if it provides all these things:
@@ -67,6 +67,7 @@ config ARM
select HAVE_KRETPROBES if (HAVE_KPROBES)
select HAVE_MEMBLOCK
select HAVE_MOD_ARCH_SPECIFIC
select HAVE_NMI
select HAVE_OPROFILE if (HAVE_PERF_EVENTS)
select HAVE_OPTPROBES if !THUMB2_KERNEL
select HAVE_PERF_EVENTS
@@ -644,9 +644,11 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
break;

case IPI_CPU_BACKTRACE:
printk_nmi_enter();
irq_enter();
nmi_cpu_backtrace(regs);
irq_exit();
printk_nmi_exit();
break;

default:
@@ -18,6 +18,7 @@ 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
@@ -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
@@ -70,6 +70,7 @@ 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
@@ -48,6 +48,7 @@ config MIPS
select GENERIC_SCHED_CLOCK if !CAVIUM_OCTEON_SOC
select GENERIC_CMOS_UPDATE
select HAVE_MOD_ARCH_SPECIFIC
select HAVE_NMI
select VIRT_TO_BUS
select MODULES_USE_ELF_REL if MODULES
select MODULES_USE_ELF_RELA if MODULES && 64BIT
@@ -155,6 +155,7 @@ 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
@@ -166,6 +166,7 @@ config S390
select TTY
select VIRT_CPU_ACCOUNTING
select VIRT_TO_BUS
select HAVE_NMI


config SCHED_OMIT_FRAME_POINTER
@@ -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
@@ -79,6 +79,7 @@ config SPARC64
select NO_BOOTMEM
select HAVE_ARCH_AUDITSYSCALL
select ARCH_SUPPORTS_ATOMIC_RMW
select HAVE_NMI

config ARCH_DEFCONFIG
string
@@ -30,6 +30,7 @@ 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
@@ -131,6 +131,7 @@ 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
@@ -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)
@@ -61,6 +61,7 @@ extern void irq_exit(void);

#define nmi_enter() \
do { \
printk_nmi_enter(); \
lockdep_off(); \
ftrace_nmi_enter(); \
BUG_ON(in_nmi()); \
@@ -77,6 +78,7 @@ extern void irq_exit(void);
preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET); \
ftrace_nmi_exit(); \
lockdep_on(); \
printk_nmi_exit(); \
} while (0)

#endif /* LINUX_HARDIRQ_H */
@@ -129,7 +129,4 @@ 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 */
@@ -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)
@@ -1454,6 +1454,11 @@ 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

config BUG
bool "BUG() support" if EXPERT
default y
@@ -569,6 +569,7 @@ 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();
@@ -1,2 +1,3 @@
obj-y = printk.o
obj-$(CONFIG_PRINTK_NMI) += nmi.o
obj-$(CONFIG_A11Y_BRAILLE_CONSOLE) += braille.o
@@ -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 */
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