[PATCH next v4 0/2] introduce printk cpu lock

[John Ogness]

Hello,

While working on removing the safe buffers for printk [0] we
stumbled on a cpu-reentrant spinning lock used by dump_stack(). This
type of lock (dubbed a cpu lock) will cause deadlock risks once we
introduce atomic consoles because atomic consoles also need such a
lock.

Although we are not yet ready to introduce atomic consoles, this is
an appropriate time to provide an official cpu lock to be used for
all things relating to printk (including the atomic consoles, once
they are introduced).

An example of cpu lock usage for atomic consoles can be found in the
PREEMPT_RT tree, such as the serial8250 implementation [1] of an
atomic console. (In PREEMPT_RT the printk cpu lock function was
named console_atomic_lock/_unlock.)

This series is against next-20210616.

v3 can be found here [2].

Changes since v3:

- Change @printk_cpulock_nested from a boolean to an atomic counter
  in order to track multiple levels of nesting.

- Correctly move over the original dump_stack() semantics by using
  atomic_set() to unlock.

- Move commit message note about disabling interrupts to the first
  patch.

John Ogness

[0] https://lore.kernel.org/lkml/YGW63%2FelFr%2FgYW1u@alley
[1] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/drivers/tty/serial?h=v5.12-rc3-rt3&id=1aaca59df62710647243a3574fc27d93c2bd0e6c
[2] https://lore.kernel.org/lkml/20210615174947.32057-1-john.ogness@xxxxxxxxxxxxx

John Ogness (2):
  lib/dump_stack: move cpu lock to printk.c
  printk: fix cpu lock ordering

 include/linux/printk.h |  41 +++++++++++++++
 kernel/printk/printk.c | 116 +++++++++++++++++++++++++++++++++++++++++
 lib/dump_stack.c       |  38 +-------------
 3 files changed, 159 insertions(+), 36 deletions(-)

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2.20.1