[PATCH printk v4 0/6] printk: remove safe buffers

[John Ogness]

Hi,

Here is v4 of a series to remove the safe buffers. v3 can be
found here [0]. The safe buffers are no longer needed because
messages can be stored directly into the log buffer from any
context.

However, the safe buffers also provided a form of recursion
protection. For that reason, explicit recursion protection is
implemented for this series.

The safe buffers also implicitly provided serialization
between multiple CPUs executing in NMI context. This was
particularly necessary for the nmi_backtrace() output. This
serializiation is now preserved by using the printk cpulock.

With the removal of the safe buffers, there is no need for
extra NMI enter/exit tracking. So this is also removed
(which includes removing the config option CONFIG_PRINTK_NMI).

And finally, there are a few places in the kernel that need to
specify code blocks where all printk calls are to be deferred
printing. Previously the NMI tracking API was being (mis)used
for this purpose. This series introduces an official and
explicit interface for such cases. (Note that all deferred
printing will be removed anyway, once printing kthreads are
introduced.)

Changes since v3:

- Remove safe context tracking in vprintk().

- Add safe context tracking for @console_owner usage since that
  is also a component of the printing code.

- Refactor syslog_print() so that it is easier to understand
  and follow the locking logic.

- Introduce printk_deferred_enter/exit functions to be used by
  code that needs to specify code block where all printk calls
  are to be deferred printing.

John Ogness

[0] https://lore.kernel.org/lkml/20210624111148.5190-1-john.ogness@xxxxxxxxxxxxx

John Ogness (6):
  lib/nmi_backtrace: explicitly serialize banner and regs
  printk: track/limit recursion
  printk: remove safe buffers
  printk: remove NMI tracking
  printk: convert @syslog_lock to mutex
  printk: syslog: close window between wait and read

 arch/arm/kernel/smp.c          |   4 +-
 arch/powerpc/kernel/traps.c    |   1 -
 arch/powerpc/kernel/watchdog.c |   5 -
 arch/powerpc/kexec/crash.c     |   2 +-
 include/linux/hardirq.h        |   2 -
 include/linux/printk.h         |  41 ++--
 init/Kconfig                   |   5 -
 kernel/kexec_core.c            |   1 -
 kernel/panic.c                 |   3 -
 kernel/printk/internal.h       |  25 ---
 kernel/printk/printk.c         | 268 ++++++++++++++----------
 kernel/printk/printk_safe.c    | 364 +--------------------------------
 kernel/trace/trace.c           |   4 +-
 lib/nmi_backtrace.c            |  13 +-
 14 files changed, 194 insertions(+), 544 deletions(-)


base-commit: 70333dec446292cd896cd051d2ebd6808b328949
-- 
2.20.1