The printk ringbuffer implementation is described in the comment as
using three ringbuffers, but the current implementation uses two (desc
and data). Update the comment so it matches the code.
Fix few more known issues in the comments.
Signed-off-by: Loïc Grégoire <loicgre@gmail.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20260328021855.53956-1-loicgre@gmail.com
[pmladek@suse.com: Fixed few more issues in the comments by John Ogness.]
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Commit cc3bad11de ("printk_ringbuffer: Fix check of valid data
size when blk_lpos overflows") added sanity checking to get_data()
to avoid returning data of illegal sizes (too large or too small).
It uses the helper function data_check_size() for the check.
However, data_check_size() expects the size of the data, not the
size of the data block. get_data() is providing the size of the
data block. This means that if the data size (text_buf_size) is
at or near the maximum legal size:
sizeof(prb_data_block) + text_buf_size == DATA_SIZE(data_ring) / 2
data_check_size() will report failure because it adds
sizeof(prb_data_block) to the provided size. The sanity check in
get_data() is counting the data block header twice. The result is
that the reader fails to read the legal record.
Since get_data() subtracts the data block header size before returning,
move the sanity check to after the subtraction.
Luckily printk() is not vulnerable to this problem because
truncate_msg() limits printk-messages to 1/4 of the ringbuffer.
Indeed, by adjusting the printk_ringbuffer KUnit test, which does not
use printk() and its truncate_msg() check, it is easy to see that the
reader fails and the WARN_ON is triggered.
Fixes: cc3bad11de ("printk_ringbuffer: Fix check of valid data size when blk_lpos overflows")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20260326133809.8045-1-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
The decision whether some more space is needed is tricky in the printk
ring buffer code:
1. The given lpos values might overflow. A subtraction must be used
instead of a simple "lower than" check.
2. Another CPU might reuse the space in the mean time. It can be
detected when the subtraction is bigger than DATA_SIZE(data_ring).
3. There is exactly enough space when the result of the subtraction
is zero. But more space is needed when the result is exactly
DATA_SIZE(data_ring).
Add a helper function to make sure that the check is done correctly
in all situations. Also it helps to make the code consistent and
better documented.
Suggested-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/87tsz7iea2.fsf@jogness.linutronix.de
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20251107194720.1231457-3-pmladek@suse.com
[pmladek@suse.com: Updated wording as suggested by John]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Previously, data blocks that perfectly fit the data ring buffer would
get wrapped around to the beginning for no reason since the calculated
offset of the next data block would belong to the next wrap. Since this
offset is not actually part of the data block, but rather the offset of
where the next data block is going to start, there is no reason to
include it when deciding whether the current block fits the buffer.
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Tested-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20250905144152.9137-2-d-tatianin@yandex-team.ru
[pmladek@suse.com: Updated indentation.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Pull printk updates from Petr Mladek:
- Add KUnit test for the printk ring buffer
- Fix the check of the maximal record size which is allowed to be
stored into the printk ring buffer. It prevents corruptions of the
ring buffer.
Note that printk() is on the safe side. The messages are limited by
1kB buffer and are always small enough for the minimal log buffer
size 4kB, see CONFIG_LOG_BUF_SHIFT definition.
* tag 'printk-for-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
printk: ringbuffer: Fix data block max size check
printk: kunit: support offstack cpumask
printk: kunit: Fix __counted_by() in struct prbtest_rbdata
printk: ringbuffer: Explain why the KUnit test ignores failed writes
printk: ringbuffer: Add KUnit test
Currently data_check_size() limits data blocks to a maximum size of
the full buffer minus an ID (long integer):
max_size <= DATA_SIZE(data_ring) - sizeof(long)
However, this is not an appropriate limit due to the nature of
wrapping data blocks. For example, if a data block is larger than
half the buffer:
size = (DATA_SIZE(data_ring) / 2) + 8
and begins exactly in the middle of the buffer, then:
- the data block will wrap
- the ID will be stored at exactly half of the buffer
- the record data begins at the beginning of the buffer
- the record data ends 8 bytes _past_ exactly half of the buffer
The record overwrites itself, i.e. needs more space than the full
buffer!
Luckily printk() is not vulnerable to this problem because
truncate_msg() limits printk-messages to 1/4 of the ringbuffer.
Indeed, by adjusting the printk_ringbuffer KUnit test, which does not
use printk() and its truncate_msg() check, it is easy to see that the
ringbuffer becomes corrupted for records larger than half the buffer
size.
The corruption occurs because data_push_tail() expects it will never
be requested to push the tail beyond the head.
Avoid this problem by adjusting data_check_size() to limit record
sizes to half the buffer size. Also add WARN_ON_ONCE() before
relevant data_push_tail() calls to validate that there are no such
illegal requests. WARN_ON_ONCE() is used, rather than just adding
extra checks to data_push_tail() because it is considered a bug to
attempt such illegal actions.
Link: https://lore.kernel.org/lkml/aMLrGCQSyC8odlFZ@pathway.suse.cz
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Commit 779dbc2e78 ("printk: Avoid non-panic CPUs writing to ringbuffer")
aimed to isolate panic-related messages. However, when panic() itself
malfunctions, messages from non-panic CPUs become crucial for debugging.
While commit bcc954c6ca ("printk/panic: Allow cpu backtraces to
be written into ringbuffer during panic") enables non-panic CPU
backtraces, it may not provide sufficient diagnostic information.
Introduce the "debug_non_panic_cpus" command-line option, enabling
non-panic CPU messages to be stored in the ring buffer during a panic.
This also prevents discarding non-finalized messages from non-panic CPUs
during console flushing, providing a more comprehensive view of system
state during critical failures.
Link: https://lore.kernel.org/all/Z8cLEkqLL2IOyNIj@pathway/
Signed-off-by: Donghyeok Choe <d7271.choe@samsung.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250318022320.2428155-1-d7271.choe@samsung.com
[pmladek@suse.com: Added documentation, added module_parameter, removed printk_ prefix.]
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Normally a reader will stop once reaching a non-finalized
record. However, when a panic happens, writers from other CPUs
(or an interrupted context on the panic CPU) may have been
writing a record and were unable to finalize it. The panic CPU
will reserve/commit/finalize its panic records, but these will
be located after the non-finalized records. This results in
panic() not flushing the panic messages.
Extend _prb_read_valid() to skip over non-finalized records if
on the panic CPU.
Fixes: 896fbe20b4 ("printk: use the lockless ringbuffer")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-11-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
Currently pr_flush() will only wait for records that were
available to readers at the time of the call (using
prb_next_seq()). But there may be more records (non-finalized)
that have following finalized records. pr_flush() should wait
for these to print as well. Particularly because any trailing
finalized records may be the messages that the calling context
wants to ensure are printed.
Add a new ringbuffer function prb_next_reserve_seq() to return
the sequence number following the most recently reserved record.
This guarantees that pr_flush() will wait until all current
printk() messages (completed or in progress) have been printed.
Fixes: 3b604ca812 ("printk: add pr_flush()")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-10-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
With the lockless ringbuffer, it is allowed that multiple
CPUs/contexts write simultaneously into the buffer. This creates
an ambiguity as some writers will finalize sooner.
The documentation for the prb_read functions is not clear as it
refers to "not yet written" and "no data available". Clarify the
return values and language to be in terms of the reader: records
available for reading.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-9-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
For empty line records, no data blocks are created. Instead,
these valid records are identified by special logical position
values (in fields of @prb_desc.text_blk_lpos).
Currently the macro NO_LPOS is used for empty line records.
This name is confusing because it does not imply _why_ there is
no data block.
Rename NO_LPOS to EMPTY_LINE_LPOS so that it is clear why there
is no data block.
Also add comments explaining the use of EMPTY_LINE_LPOS as well
as clarification to the values used to represent data-less
blocks.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-6-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
Commit f244b4dc53 ("printk: ringbuffer: Improve
prb_next_seq() performance") introduced an optimization for
prb_next_seq() by using best-effort to track recently finalized
records. However, the order of finalization does not
necessarily match the order of the records. The optimization
changed prb_next_seq() to return inconsistent results, possibly
yielding sequence numbers that are not available to readers
because they are preceded by non-finalized records or they are
not yet visible to the reader CPU.
Rather than simply best-effort tracking recently finalized
records, force the committing writer to read records and
increment the last "contiguous block" of finalized records. In
order to do this, the sequence number instead of ID must be
stored because ID's cannot be directly compared.
A new memory barrier pair is introduced to guarantee that a
reader can always read the records up until the sequence number
returned by prb_next_seq() (unless the records have since
been overwritten in the ringbuffer).
This restores the original functionality of prb_next_seq()
while also keeping the optimization.
For 32bit systems, only the lower 32 bits of the sequence
number are stored. When reading the value, it is expanded to
the full 64bit sequence number using the 32bit seq macros,
which fold in the value returned by prb_first_seq().
Fixes: f244b4dc53 ("printk: ringbuffer: Improve prb_next_seq() performance")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-5-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
Note: This change only applies to 32bit architectures. On 64bit
architectures the macros are NOPs.
Currently prb_next_seq() is used as the base for the 32bit seq
macros __u64seq_to_ulseq() and __ulseq_to_u64seq(). However, in
a follow-up commit, prb_next_seq() will need to make use of the
32bit seq macros.
Use prb_first_seq() as the base for the 32bit seq macros instead
because it is guaranteed to return 64bit sequence numbers without
relying on any 32bit seq macros.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-4-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
prb_next_seq() always iterates from the first known sequence number.
In the worst case, it might loop 8k times for 256kB buffer,
15k times for 512kB buffer, and 64k times for 2MB buffer.
It was reported that polling and reading using syslog interface
might occupy 50% of CPU.
Speedup the search by storing @id of the last finalized descriptor.
The loop is still needed because the @id is stored and read in the best
effort way. An atomic variable is used to keep the @id consistent.
But the stores and reads are not serialized against each other.
The descriptor could get reused in the meantime. The related sequence
number will be used only when it is still valid.
An invalid value should be read _only_ when there is a flood of messages
and the ringbuffer is rapidly reused. The performance is the least
problem in this case.
Reported-by: Chunlei Wang <chunlei.wang@mediatek.com>
Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/1642770388-17327-1-git-send-email-quic_mojha@quicinc.com
Link: https://lore.kernel.org/lkml/YXlddJxLh77DKfIO@alley/T/#m43062e8b2a17f8dbc8c6ccdb8851fb0dbaabbb14
Counting text lines in a record simply involves counting the number
of newline characters (+1). However, it is searching the full data
block for newline characters, even though the text data can be (and
often is) a subset of that area. Since the extra area in the data
block was never initialized, the result is that extra newlines may
be seen and counted.
Restrict newline searching to the text data length.
Fixes: b6cf8b3f33 ("printk: add lockless ringbuffer")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210113144234.6545-1-john.ogness@linutronix.de
Pull printk fixes from Petr Mladek:
- do not lose trailing newline in pr_cont() calls
- two trivial fixes for a dead store and a config description
* tag 'printk-for-5.10-rc6-fixup' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
printk: finalize records with trailing newlines
printk: remove unneeded dead-store assignment
init/Kconfig: Fix CPU number in LOG_CPU_MAX_BUF_SHIFT description
Loïc GrégoireandPetr Mladek
John OgnessandPetr Mladek
Daniil TatianinandPetr Mladek
Linus Torvalds
Jinchao WangandAndrew Morton
Thomas WeißschuhandPetr Mladek
Donghyeok ChoeandPetr Mladek
Kees CookandPetr Mladek
Wang HonghuiandPetr Mladek