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Sergey Senozhatsky 4a5c9ae67b printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 
commit ab6f762f0f upstream.

printk_deferred(), similarly to printk_safe/printk_nmi, does not
immediately attempt to print a new message on the consoles, avoiding
calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
which potentially can deadlock the system.

Those printk() flavors, instead, rely on per-CPU flush irq_work to print
messages from safer contexts.  For same reasons (recursive scheduler or
timekeeping calls) printk() uses per-CPU irq_work in order to wake up
user space syslog/kmsg readers.

However, only printk_safe/printk_nmi do make sure that per-CPU areas
have been initialised and that it's safe to modify per-CPU irq_work.
This means that, for instance, should printk_deferred() be invoked "too
early", that is before per-CPU areas are initialised, printk_deferred()
will perform illegal per-CPU access.

Lech Perczak [0] reports that after commit 1b710b1b10 ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.

The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).

Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.

Link: https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/
Reported-by: Lech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Jann Horn <jannh@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 		2020-05-02 08:48:42 +02:00
Documentation USB: hub: Revert commit bd0e6c9614 ("usb: hub: try old enumeration scheme first for high speed devices") 2020-04-29 16:33:14 +02:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch s390/mm: fix page table upgrade vs 2ndary address mode accesses 2020-04-29 16:33:25 +02:00
block block, bfq: invoke flush_idle_tree after reparent_active_queues in pd_offline 2020-04-23 10:36:26 +02:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto crypto: rng - Fix a refcounting bug in crypto_rng_reset() 2020-04-17 10:50:16 +02:00
drivers remoteproc: Fix wrong rvring index computation 2020-05-02 08:48:41 +02:00
fs ubifs: Fix ubifs_tnc_lookup() usage in do_kill_orphans() 2020-05-02 08:48:42 +02:00
include printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 2020-05-02 08:48:42 +02:00
init printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 2020-05-02 08:48:42 +02:00
ipc ipc/util.c: sysvipc_find_ipc() should increase position index 2020-04-29 16:33:00 +02:00
kernel printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 2020-05-02 08:48:42 +02:00
lib lib/raid6/test: fix build on distros whose /bin/sh is not bash 2020-04-29 16:33:00 +02:00
mm mm/ksm: fix NULL pointer dereference when KSM zero page is enabled 2020-04-29 16:33:15 +02:00
net SUNRPC: Fix backchannel RPC soft lockups 2020-04-29 16:33:18 +02:00
samples vmalloc: fix remap_vmalloc_range() bounds checks 2020-04-29 16:33:14 +02:00
scripts kconfig: qconf: Fix a few alignment issues 2020-04-29 16:33:00 +02:00
security KEYS: Avoid false positive ENOMEM error on key read 2020-04-29 16:33:11 +02:00
sound ASoC: dapm: fixup dapm kcontrol widget 2020-04-29 16:33:18 +02:00
tools tools/vm: fix cross-compile build 2020-04-29 16:33:15 +02:00
usr initramfs: restore default compression behavior 2020-04-08 09:08:38 +02:00
virt KVM: Check for a bad hva before dropping into the ghc slow path 2020-03-05 16:43:48 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig
…
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Modules updates for v5.4 2019-09-22 10:34:46 -07:00
.mailmap ARM: SoC fixes 2019-11-10 13:41:59 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer 2019-10-10 08:12:51 -07:00
Kbuild kbuild: do not descend to ./Kbuild when cleaning 2019-08-21 21:03:58 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 17:22:19 +01:00
Makefile Linux 5.4.36 2020-04-29 16:33:25 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

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.