[PATCH v2 0/3] printk: replace ringbuffer

John Ogness
2020-05-01T09:40:45Z

Hello,

Here is a v2 for the first series to rework the printk subsystem. The
v1 and history are here [0]. This first series only replaces the
existing ringbuffer implementation. No locking is removed. No
semantics/behavior of printk are changed.

The VMCOREINFO is updated. RFC patches for the external tools
crash(8) [1] and makedumpfile(8) [2] have been submitted that allow
the new ringbuffer to be correctly read.

This series is in line with the agreements [3] made at the meeting
during LPC2019 in Lisbon, with 1 exception: support for dictionaries
will not be discontinued [4]. Dictionaries are stored in a separate
buffer so that they cannot interfere with the human-readable buffer.

The list of changes since v1:

printk_ringbuffer
=================
- documentation formatting/wording cleanup
- main memory barrier comments moved from memory accesses to
memory barriers
- remove all symbol exporting of printk_ringbuffer functions
- add prb_count_lines() for line counting so printk.c can also use
this functionality
- add prb_read_valid_info() to just read record meta-data
- remove @line_count out of printk_record (there is now an interface
for line counting)
- to_block(): remove intermediate (char *) typecast
- data_alloc(): use WRITE_ONCE for writing block ID
- data_push_tail(): fix test condition for tail LPOS already pushed
- data_push_tail(): continue loop rather than try cmpxchg() if
tail LPOS was pushed
- data_push_tail(): add memory barrier before pushing tail LPOS so
readers can recognize if data has expired while reading the
descriptor
- data_make_reusable(): added memory barrier and logic to protect
against invalid read of block ID
- data_alloc(): add memory barrier to guarantee the tail ID is stored
before updating the block ID
- desc_reserve(): add memory barrier before checking ID to make sure
the head ID is read before the tail ID
- desc_reserve(): move memory barrier to before the head ID is pushed
to guarantee the tail ID is stored before the head ID

printk.c
========
- remove static syslog_record/console_record (use dynamic allocation
and in-place buffer manipulation instead)
- reduce expected average message size from 64 to 32
- add printk_record initializers to simplify reader and writer code
- fix wrong return value when log_store() truncates
- add full VMCOREINFO needed by makedumpfile(8)
- change record_print_text() to add prefix data to the buffer
in-place instead of requiring a 2nd buffer
- add helper to calculate text size when prefix added:
get_record_text_size() (used together with prb_count_lines() or
prb_read_valid_info() to get full text size, rather than abusing
record_print_text() for this purpose)
- syslog_print_all(): fix missing break if copy_to_user() failed
- moved "messages dropped" printing to call_console_drivers()

John Ogness

[0] https://lkml.kernel.org/r/20200128161948.8524-1-john.ogness@linutronix.de
[1] https://www.redhat.com/archives/crash-utility/2020-April/msg00080.html
[2] https://lists.infradead.org/pipermail/kexec/2020-April/024906.html
[3] https://lkml.kernel.org/r/87k1acz5rx.fsf@linutronix.de
[4] https://lkml.kernel.org/r/20191007120134.ciywr3wale4gxa6v@pathway.suse.cz

John Ogness (3):
crash: add VMCOREINFO macro for anonymous structs
printk: add lockless buffer
printk: use the lockless ringbuffer

include/linux/crash_core.h | 3 +
include/linux/kmsg_dump.h | 2 -
kernel/printk/Makefile | 1 +
kernel/printk/printk.c | 938 +++++++++--------
kernel/printk/printk_ringbuffer.c | 1626 +++++++++++++++++++++++++++++
kernel/printk/printk_ringbuffer.h | 369 +++++++
6 files changed, 2491 insertions(+), 448 deletions(-)
create mode 100644 kernel/printk/printk_ringbuffer.c
create mode 100644 kernel/printk/printk_ringbuffer.h

--
2.20.1