[RFC][PATCHv3 0/6] printk: use printk_safe to handle printk() recursive calls

Sergey Senozhatsky
2016-10-18T15:42:15Z

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 printk_safe 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-20161018]]

v3: (review by Petr)
-- renamed to printk_safe
-- !NMI config build fix
-- report lost messages for both printk_sae and printk_nmi
-- dropped recursion reporting patch
-- etc.

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 (6):
printk: use vprintk_func in vprintk()
printk: rename nmi.c and exported api
printk: introduce per-cpu safe_print seq buffer
printk: report lost messages in printk safe/nmi contexts
printk: use printk_safe buffers
printk: remove zap_locks() function

arch/arm/kernel/smp.c | 4 +-
include/linux/hardirq.h | 4 +-
include/linux/printk.h | 29 ++--
init/Kconfig | 16 ++-
init/main.c | 2 +-
kernel/kexec_core.c | 2 +-
kernel/panic.c | 4 +-
kernel/printk/Makefile | 3 +-
kernel/printk/internal.h | 65 +++++----
kernel/printk/printk.c | 116 ++++------------
kernel/printk/{nmi.c => printk_safe.c} | 239 +++++++++++++++++++++++++--------
lib/nmi_backtrace.c | 2 +-
12 files changed, 277 insertions(+), 209 deletions(-)
rename kernel/printk/{nmi.c => printk_safe.c} (54%)

--
2.10.1.382.ga23ca1b