The commit 719f6a70 ("printk: Use the main logbuf in NMI
when logbuf_lock is available") brought back the possible deadlocks
in printk() and NMI.

The check of logbuf_lock is done only in printk_nmi_enter() to prevent
mixed output. But another CPU might take the lock later, enter NMI, and:

      + Both NMIs might be serialized by yet another lock, for example,
	the one in nmi_cpu_backtrace().

      + The other CPU might get stopped in NMI, see smp_send_stop()
	in panic().

The only safe solution is to use trylock when storing the message
into the main log-buffer. It might cause reordering when some lines
go to the main lock buffer directly and others are delayed via
the per-CPU buffer. It means that it is not useful in general.

This patch replaces the problematic NMI deferred context with NMI
direct context. It can be used to mark a code that might produce
many messages in NMI and the risk of losing them is more critical
than problems with eventual reordering.

The context is then used when dumping trace buffers on oops. It was
the primary motivation for the original fix. Also the reordering is
even smaller issue there because some traces have their own time stamps.

Finally, nmi_cpu_backtrace() need not longer be serialized because
it will always us the per-CPU buffers again.

Fixes: 719f6a70 ("printk: Use the main logbuf in NMI when logbuf_lock is available")
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20180627142028.11259-1-pmladek@suse.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Acked-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NPetr Mladek <pmladek@suse.com>

The commit 42a0bb3f ("printk/nmi: generic solution for safe
printk in NMI") caused that printk stores messages into a temporary
buffer in NMI context.

The buffer is per-CPU and therefore the size is rather limited.
It works quite well for NMI backtraces. But there are longer logs
that might get printed in NMI context, for example, lockdep
warnings, ftrace_dump_on_oops.

The temporary buffer is used to avoid deadlocks caused by
logbuf_lock. Also it is needed to avoid races with the other
temporary buffer that is used when PRINTK_SAFE_CONTEXT is entered.
But the main buffer can be used in NMI if the lock is available
and we did not interrupt PRINTK_SAFE_CONTEXT.

The lock is checked using raw_spin_is_locked(). It might cause
false negatives when the lock is taken on another CPU and
this CPU is in the safe context from other reasons. Note that
the safe context is used also to get console semaphore or when
calling console drivers. For this reason, we do the check in
printk_nmi_enter(). It makes the handling consistent for
the entire NMI handler and avoids reshuffling of the messages.

The patch also defines special printk context that allows
to use printk_deferred() in NMI. Note that we could not flush
the messages to the consoles because console drivers might use
many other internal locks.

The newly created vprintk_deferred() disables the preemption
only around the irq work handling. It is needed there to keep
the consistency between the two per-CPU variables. But there
is no reason to disable preemption around vprintk_emit().

Finally, the patch puts back explicit serialization of the NMI
backtraces from different CPUs. It was removed by the
commit a9edc880 ("x86/nmi: Perform a safe
NMI stack trace on all CPUs"). It was not needed because
the flushing of the temporary per-CPU buffers was serialized.

Link: http://lkml.kernel.org/r/1493912763-24873-1-git-send-email-pmladek@suse.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King <rack+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: x86@kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NPetr Mladek <pmladek@suse.com>

Account lost messages in pritk-safe and printk-safe-nmi
contexts and report those numbers during printk_safe_flush().

The patch also moves lost message counter to struct
`printk_safe_seq_buf' instead of having dedicated static
counters - this simplifies the code.

Link: http://lkml.kernel.org/r/20161227141611.940-6-sergey.senozhatsky@gmail.com
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Calvin Owens <calvinowens@fb.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NPetr Mladek <pmladek@suse.com>

This patch extends the idea of NMI per-cpu buffers to regions
that may cause recursive printk() calls and possible deadlocks.
Namely, printk() can't handle printk calls from schedule code
or printk() calls from lock debugging code (spin_dump() for instance);
because those may be called with `sem->lock' already taken or any
other `critical' locks (p->pi_lock, etc.). An example of deadlock
can be

 vprintk_emit()
  console_unlock()
   up()                        << raw_spin_lock_irqsave(&sem->lock, flags);
    wake_up_process()
     try_to_wake_up()
      ttwu_queue()
       ttwu_activate()
        activate_task()
         enqueue_task()
          enqueue_task_fair()
           cfs_rq_of()
            task_of()
             WARN_ON_ONCE(!entity_is_task(se))
              vprintk_emit()
               console_trylock()
                down_trylock()
                 raw_spin_lock_irqsave(&sem->lock, flags)
                 ^^^^ deadlock

and some other cases.

Just like in NMI implementation, the solution uses a per-cpu
`printk_func' pointer to 'redirect' printk() calls to a 'safe'
callback, that store messages in a per-cpu buffer and flushes
them back to logbuf buffer later.

Usage example:

 printk()
  printk_safe_enter_irqsave(flags)
  //
  //  any printk() call from here will endup in vprintk_safe(),
  //  that stores messages in a special per-CPU buffer.
  //
  printk_safe_exit_irqrestore(flags)

The 'redirection' mechanism, though, has been reworked, as suggested
by Petr Mladek. Instead of using a per-cpu @print_func callback we now
keep a per-cpu printk-context variable and call either default or nmi
vprintk function depending on its value. printk_nmi_entrer/exit and
printk_safe_enter/exit, thus, just set/celar corresponding bits in
printk-context functions.

The patch only adds printk_safe support, we don't use it yet.

Link: http://lkml.kernel.org/r/20161227141611.940-4-sergey.senozhatsky@gmail.com
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Calvin Owens <calvinowens@fb.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NPetr Mladek <pmladek@suse.com>
Reviewed-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

This reverts commit 874f9c7d.

Geert Uytterhoeven reports:
 "This change seems to have an (unintendent?) side-effect.

  Before, pr_*() calls without a trailing newline characters would be
  printed with a newline character appended, both on the console and in
  the output of the dmesg command.

  After this commit, no new line character is appended, and the output
  of the next pr_*() call of the same type may be appended, like in:

    - Truncating RAM at 0x0000000040000000-0x00000000c0000000 to -0x0000000070000000
    - Ignoring RAM at 0x0000000200000000-0x0000000240000000 (!CONFIG_HIGHMEM)
    + Truncating RAM at 0x0000000040000000-0x00000000c0000000 to -0x0000000070000000Ignoring RAM at 0x0000000200000000-0x0000000240000000 (!CONFIG_HIGHMEM)"

Joe Perches says:
 "No, that is not intentional.

  The newline handling code inside vprintk_emit is a bit involved and
  for now I suggest a revert until this has all the same behavior as
  earlier"
Reported-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Requested-by: NJoe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Using functions instead of macros can reduce overall code size by
eliminating unnecessary "KERN_SOH<digit>" prefixes from format strings.

  defconfig x86-64:

  $ size vmlinux*
     text    data     bss      dec     hex  filename
  10193570 4331464 1105920 15630954  ee826a vmlinux.new
  10192623 4335560 1105920 15634103  ee8eb7 vmlinux.old

As the return value are unimportant and unused in the kernel tree, these
new functions return void.

Miscellanea:

 - change pr_<level> macros to call new __pr_<level> functions
 - change vprintk_nmi and vprintk_default to add LOGLEVEL_<level> argument

[akpm@linux-foundation.org: fix LOGLEVEL_INFO, per Joe]
Link: http://lkml.kernel.org/r/e16cc34479dfefcae37c98b481e6646f0f69efc3.1466718827.git.joe@perches.comSigned-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

In NMI context, printk() messages are stored into per-CPU buffers to
avoid a possible deadlock.  They are normally flushed to the main ring
buffer via an IRQ work.  But the work is never called when the system
calls panic() in the very same NMI handler.

This patch tries to flush NMI buffers before the crash dump is
generated.  In this case it does not risk a double release and bails out
when the logbuf_lock is already taken.  The aim is to get the messages
into the main ring buffer when possible.  It makes them better
accessible in the vmcore.

Then the patch tries to flush the buffers second time when other CPUs
are down.  It might be more aggressive and reset logbuf_lock.  The aim
is to get the messages available for the consequent kmsg_dump() and
console_flush_on_panic() calls.

The patch causes vprintk_emit() to be called even in NMI context again.
But it is done via printk_deferred() so that the console handling is
skipped.  Consoles use internal locks and we could not prevent a
deadlock easily.  They are explicitly called later when the crash dump
is not generated, see console_flush_on_panic().
Signed-off-by: NPetr Mladek <pmladek@suse.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: David Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

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.

[akpm@linux-foundation.org: whitespace fixlet]
Signed-off-by: NPetr 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>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

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 a9edc880 ("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: NPetr Mladek <pmladek@suse.com>
Suggested-by: NPeter Zijlstra <peterz@infradead.org>
Suggested-by: NSteven 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: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>