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Steven Rostedt (VMware) c9fd0ab39f tracing: Do no increment trace_clock_global() by one commit `89529d8b8f` upstream. The trace_clock_global() tries to make sure the events between CPUs is somewhat in order. A global value is used and updated by the latest read of a clock. If one CPU is ahead by a little, and is read by another CPU, a lock is taken, and if the timestamp of the other CPU is behind, it will simply use the other CPUs timestamp. The lock is also only taken with a "trylock" due to tracing, and strange recursions can happen. The lock is not taken at all in NMI context. In the case where the lock is not able to be taken, the non synced timestamp is returned. But it will not be less than the saved global timestamp. The problem arises because when the time goes "backwards" the time returned is the saved timestamp plus 1. If the lock is not taken, and the plus one to the timestamp is returned, there's a small race that can cause the time to go backwards! CPU0 CPU1 ---- ---- trace_clock_global() { ts = clock() [ 1000 ] trylock(clock_lock) [ success ] global_ts = ts; [ 1000 ] <interrupted by NMI> trace_clock_global() { ts = clock() [ 999 ] if (ts < global_ts) ts = global_ts + 1 [ 1001 ] trylock(clock_lock) [ fail ] return ts [ 1001] } unlock(clock_lock); return ts; [ 1000 ] } trace_clock_global() { ts = clock() [ 1000 ] if (ts < global_ts) [ false 1000 == 1000 ] trylock(clock_lock) [ success ] global_ts = ts; [ 1000 ] unlock(clock_lock) return ts; [ 1000 ] } The above case shows to reads of trace_clock_global() on the same CPU, but the second read returns one less than the first read. That is, time when backwards, and this is not what is allowed by trace_clock_global(). This was triggered by heavy tracing and the ring buffer checker that tests for the clock going backwards: Ring buffer clock went backwards: 20613921464 -> 20613921463 ------------[ cut here ]------------ WARNING: CPU: 2 PID: 0 at kernel/trace/ring_buffer.c:3412 check_buffer+0x1b9/0x1c0 Modules linked in: [..] [CPU: 2]TIME DOES NOT MATCH expected:20620711698 actual:20620711697 delta:6790234 before:20613921463 after:20613921463 [20613915818] PAGE TIME STAMP [20613915818] delta:0 [20613915819] delta:1 [20613916035] delta:216 [20613916465] delta:430 [20613916575] delta:110 [20613916749] delta:174 [20613917248] delta:499 [20613917333] delta:85 [20613917775] delta:442 [20613917921] delta:146 [20613918321] delta:400 [20613918568] delta:247 [20613918768] delta:200 [20613919306] delta:538 [20613919353] delta:47 [20613919980] delta:627 [20613920296] delta:316 [20613920571] delta:275 [20613920862] delta:291 [20613921152] delta:290 [20613921464] delta:312 [20613921464] delta:0 TIME EXTEND [20613921464] delta:0 This happened more than once, and always for an off by one result. It also started happening after commit `aafe104aa9` was added. Cc: stable@vger.kernel.org Fixes: `aafe104aa9` ("tracing: Restructure trace_clock_global() to never block") Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2021-06-23 14:42:50 +02:00
arch	kvm: LAPIC: Restore guard to prevent illegal APIC register access	2021-06-23 14:42:41 +02:00
block	blk-mq: Swap two calls in blk_mq_exit_queue()	2021-05-19 10:13:14 +02:00
certs	certs: Fix blacklist flag type confusion	2021-03-04 11:37:59 +01:00
crypto	async_xor: check src_offs is not NULL before updating it	2021-06-16 12:01:40 +02:00
Documentation	ASoC: meson: gx-card: fix sound-dai dt schema	2021-06-16 12:01:45 +02:00
drivers	usb: chipidea: imx: Fix Battery Charger 1.2 CDP detection	2021-06-23 14:42:50 +02:00
fs	fanotify: fix copy_event_to_user() fid error clean up	2021-06-23 14:42:41 +02:00
include	mm: relocate 'write_protect_seq' in struct mm_struct	2021-06-23 14:42:49 +02:00
init	pid: take a reference when initializing `cad_pid`	2021-06-10 13:39:26 +02:00
ipc	ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry	2021-05-26 12:06:54 +02:00
kernel	tracing: Do no increment trace_clock_global() by one	2021-06-23 14:42:50 +02:00
lib	lib/lz4: explicitly support in-place decompression	2021-06-10 13:39:29 +02:00
LICENSES	LICENSES/deprecated: add Zlib license text	2020-09-16 14:33:49 +02:00
mm	mm/memory-failure: make sure wait for page writeback in memory_failure	2021-06-23 14:42:40 +02:00
net	can: j1939: fix Use-after-Free, hold skb ref while in use	2021-06-23 14:42:50 +02:00
samples	samples: vfio-mdev: fix error handing in mdpy_fb_probe()	2021-06-10 13:39:15 +02:00
scripts	scripts/clang-tools: switch explicitly to Python 3	2021-06-03 09:00:52 +02:00
security	KEYS: trusted: Fix memory leak on object td	2021-05-19 10:12:50 +02:00
sound	ASoC: qcom: lpass-cpu: Fix pop noise during audio capture begin	2021-06-23 14:42:49 +02:00
tools	ipv4: Fix device used for dst_alloc with local routes	2021-06-23 14:42:45 +02:00
usr	Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs	2020-08-07 13:29:39 -07:00
virt	Revert "irqbypass: do not start cons/prod when failed connect"	2021-06-03 09:00:34 +02:00
.clang-format	RDMA 5.10 pull request	2020-10-17 11:18:18 -07:00
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore	kbuild: generate Module.symvers only when vmlinux exists	2021-05-19 10:12:59 +02:00
.mailmap	mailmap: add two more addresses of Uwe Kleine-König	2020-12-06 10:19:07 -08:00
COPYING
CREDITS	MAINTAINERS: Move Jason Cooper to CREDITS	2020-11-30 10:20:34 +01:00
Kbuild
Kconfig	kbuild: ensure full rebuild when the compiler is updated	2020-05-12 13:28:33 +09:00
MAINTAINERS	f2fs: move ioctl interface definitions to separated file	2021-05-19 10:13:00 +02:00
Makefile	Linux 5.10.45	2021-06-18 10:00:06 +02:00
README

README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.