[RFC][PATCH 0/7] printk: use alt_printk to handle printk() recursive calls

[Sergey Senozhatsky]

	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.

Deadlocks 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);

and some other.

[[against next-20160927]]


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: drop vprintk_func function
  printk: use alternative printk buffers
  printk: new printk() recursion detection

 include/linux/printk.h     |  14 +-
 init/Kconfig               |  16 +-
 init/main.c                |   2 +-
 kernel/kexec_core.c        |   2 +-
 kernel/panic.c             |   4 +-
 kernel/printk/Makefile     |   2 +-
 kernel/printk/alt_printk.c | 432 +++++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/internal.h   |  26 ++-
 kernel/printk/nmi.c        | 268 ----------------------------
 kernel/printk/printk.c     |  67 +++----
 lib/nmi_backtrace.c        |   2 +-
 11 files changed, 495 insertions(+), 340 deletions(-)
 create mode 100644 kernel/printk/alt_printk.c
 delete mode 100644 kernel/printk/nmi.c

-- 
2.10.0.372.g6fe1b14