Hello,

        RFC

        This patch set extends a lock-less NMI per-cpu buffers idea to
handle recursive printk() calls. The basic mechanism is pretty much the
same -- at the beginning of a deadlock-prone section we switch to lock-less
printk callback, and return back to a default printk implementation at the
end; the messages are getting flushed to a logbuf buffer from a safer
context.

Deadlock scenarios that alt_printk can handle:

a) printk recursion from logbuf_lock spin_lock section in printk()
  printk()
    raw_spin_lock(&logbuf_lock);
    WARN_ON(1);
    raw_spin_unlock(&logbuf_lock);

b) printk from sem->lock spin_lock section
  printk()
    console_trylock()
      down_trylock()
        raw_spin_lock_irqsave(&sem->lock, flags);
        WARN_ON(1);
        raw_spin_unlock_irqrestore(&sem->lock, flags);

c) printk from logbuf_lock spin_lock section in console_unlock()
  printk()
    console_unlock()
      raw_spin_lock(&logbuf_lock);
      WARN_ON(1);
      raw_spin_unlock(&logbuf_lock);

d) printk from ->pi_lock from semaphore up
  printk()
    console_unlock()
      up()
        try_to_wake_up()
          raw_spin_lock_irqsave(&p->pi_lock, flags);
          WARN_ON(1);
          raw_spin_unlock_irqrestore(&p->pi_lock, flags);

[[against next-20160929]]

v2:
-- fix build error on !NMI configs, reported by Fengguang
-- reworked the series based on Petr's feedback
-- added a new patch to drop zap_locks()

Sergey Senozhatsky (7):
  printk: use vprintk_func in vprintk()
  printk: rename nmi.c and exported api
  printk: introduce per-cpu alt_print seq buffer
  printk: make alt_printk available when config printk set
  printk: use alternative printk buffers
  printk: report printk recursion from alt_printk flush
  printk: remove zap_locks() function

 include/linux/printk.h                |  21 +++-
 init/Kconfig                          |  16 +--
 init/main.c                           |   2 +-
 kernel/kexec_core.c                   |   2 +-
 kernel/panic.c                        |   4 +-
 kernel/printk/Makefile                |   2 +-
 kernel/printk/{nmi.c => alt_printk.c} | 207 +++++++++++++++++++++++++---------
 kernel/printk/internal.h              |  44 ++++----
 kernel/printk/printk.c                |  92 +++------------
 lib/nmi_backtrace.c                   |   2 +-
 10 files changed, 220 insertions(+), 172 deletions(-)
 rename kernel/printk/{nmi.c => alt_printk.c} (58%)

-- 
2.10.0.372.g6fe1b14