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Petr Mladek 42a0bb3f71 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 a9edc88093 ("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>
 		2016-05-20 17:58:30 -07:00
..
boot powerpc/86xx: Fix PCI interrupt map definition 2016-05-16 18:45:39 -05:00
configs powerpc updates for 4.6 2016-03-19 15:38:41 -07:00
crypto powerpc updates for 4.6 2016-03-19 15:38:41 -07:00
include powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
kernel exit_thread: remove empty bodies 2016-05-20 17:58:30 -07:00
kvm powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
lib powerpc/sstep: Fix emulation fall-through 2016-05-11 21:54:08 +10:00
math-emu
…
mm powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
net net: filter: make JITs zero A for SKF_AD_ALU_XOR_X 2016-01-06 00:43:52 -05:00
oprofile powerpc: Fix misspellings in comments. 2016-03-01 19:27:20 +11:00
perf powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
platforms powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
scripts powerpc/ftrace: Add Kconfig & Make glue for mprofile-kernel 2016-03-07 14:53:56 +11:00
sysdev powerpc updates for 4.7 2016-05-20 10:12:41 -07:00
xmon powerpc/xmon: Fix SPR read/write commands and add command to dump SPRs 2016-05-11 21:54:07 +10:00
Kconfig printk/nmi: generic solution for safe printk in NMI 2016-05-20 17:58:30 -07:00
Kconfig.debug powerpc/mm: Always use STRICT_MM_TYPECHECKS 2016-05-01 18:32:14 +10:00
Makefile Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/scottwood/linux into next 2016-03-14 20:05:14 +11:00
relocs_check.sh powerpc: Convert relocs_check to a shell script using grep 2015-03-23 14:47:39 +11:00