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NeilBrown 02bd7b740c fscache: fix race between enablement and dropping of object [ Upstream commit `c5a94f434c` ] It was observed that a process blocked indefintely in __fscache_read_or_alloc_page(), waiting for FSCACHE_COOKIE_LOOKING_UP to be cleared via fscache_wait_for_deferred_lookup(). At this time, ->backing_objects was empty, which would normaly prevent __fscache_read_or_alloc_page() from getting to the point of waiting. This implies that ->backing_objects was cleared after __fscache_read_or_alloc_page was was entered. When an object is "killed" and then "dropped", FSCACHE_COOKIE_LOOKING_UP is cleared in fscache_lookup_failure(), then KILL_OBJECT and DROP_OBJECT are "called" and only in DROP_OBJECT is ->backing_objects cleared. This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP and __fscache_read_or_alloc_page() can start waiting, before ->backing_objects is cleared There is some uncertainty in this analysis, but it seems to be fit the observations. Adding the wake in this patch will be handled correctly by __fscache_read_or_alloc_page(), as it checks if ->backing_objects is empty again, after waiting. Customer which reported the hang, also report that the hang cannot be reproduced with this fix. The backtrace for the blocked process looked like: PID: 29360 TASK: ffff881ff2ac0f80 CPU: 3 COMMAND: "zsh" #0 [ffff881ff43efbf8] schedule at ffffffff815e56f1 #1 [ffff881ff43efc58] bit_wait at ffffffff815e64ed #2 [ffff881ff43efc68] __wait_on_bit at ffffffff815e61b8 #3 [ffff881ff43efca0] out_of_line_wait_on_bit at ffffffff815e625e #4 [ffff881ff43efd08] fscache_wait_for_deferred_lookup at ffffffffa04f2e8f [fscache] #5 [ffff881ff43efd18] __fscache_read_or_alloc_page at ffffffffa04f2ffe [fscache] #6 [ffff881ff43efd58] __nfs_readpage_from_fscache at ffffffffa0679668 [nfs] #7 [ffff881ff43efd78] nfs_readpage at ffffffffa067092b [nfs] #8 [ffff881ff43efda0] generic_file_read_iter at ffffffff81187a73 #9 [ffff881ff43efe50] nfs_file_read at ffffffffa066544b [nfs] #10 [ffff881ff43efe70] __vfs_read at ffffffff811fc756 #11 [ffff881ff43efee8] vfs_read at ffffffff811fccfa #12 [ffff881ff43eff18] sys_read at ffffffff811fda62 #13 [ffff881ff43eff50] entry_SYSCALL_64_fastpath at ffffffff815e986e Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Sasha Levin <sashal@kernel.org>		2018-12-17 09:24:40 +01:00
arch	Revert "xen/balloon: Mark unallocated host memory as UNUSABLE"	2018-12-17 09:24:39 +01:00
block	blk-mq: punt failed direct issue to dispatch list	2018-12-08 12:59:10 +01:00
certs	export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR()	2018-08-22 23:21:44 +09:00
crypto	crypto: do not free algorithm before using	2018-12-13 09:16:21 +01:00
Documentation	x86/speculation: Provide IBPB always command line options	2018-12-05 19:32:04 +01:00
drivers	pvcalls-front: fixes incorrect error handling	2018-12-17 09:24:39 +01:00
firmware	kbuild: remove all dummy assignments to obj-	2017-11-18 11:46:06 +09:00
fs	fscache: fix race between enablement and dropping of object	2018-12-17 09:24:40 +01:00
include	pstore/ram: Correctly calculate usable PRZ bytes	2018-12-17 09:24:39 +01:00
init	Kbuild updates for v4.19 (2nd)	2018-08-25 13:40:38 -07:00
ipc	ipc/shm.c: use ERR_CAST() for shm_lock() error return	2018-10-05 16:32:04 -07:00
kernel	bpf: fix check of allowed specifiers in bpf_trace_printk	2018-12-17 09:24:35 +01:00
lib	test_firmware: fix error return getting clobbered	2018-12-13 09:16:10 +01:00
LICENSES	LICENSES: Remove CC-BY-SA-4.0 license text	2018-10-18 11:28:50 +02:00
mm	userfaultfd: shmem: UFFDIO_COPY: set the page dirty if VM_WRITE is not set	2018-12-08 12:59:08 +01:00
net	netfilter: nf_tables: deactivate expressions in rule replecement routine	2018-12-17 09:24:38 +01:00
samples	samples: disable CONFIG_SAMPLES for UML	2018-10-11 02:15:46 +09:00
scripts	unifdef: use memcpy instead of strncpy	2018-12-08 12:59:06 +01:00
security	selinux: add support for RTM_NEWCHAIN, RTM_DELCHAIN, and RTM_GETCHAIN	2018-12-08 12:59:08 +01:00
sound	ALSA: usb-audio: Add vendor and product name for Dell WD19 Dock	2018-12-17 09:24:38 +01:00
tools	tools: bpftool: fix potential NULL pointer dereference in do_load	2018-12-17 09:24:34 +01:00
usr	initramfs: move gen_initramfs_list.sh from scripts/ to usr/	2018-08-22 23:21:44 +09:00
virt	KVM: arm64: Fix caching of host MDCR_EL2 value	2018-11-13 11:08:47 -08:00
.clang-format	clang-format: Set IndentWrappedFunctionNames false	2018-08-01 18:38:51 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore	Kbuild updates for v4.17 (2nd)	2018-04-15 17:21:30 -07:00
.mailmap	libnvdimm-for-4.19_misc	2018-08-25 18:13:10 -07:00
COPYING	COPYING: use the new text with points to the license files	2018-03-23 12:41:45 -06:00
CREDITS	9p: remove Ron Minnich from MAINTAINERS	2018-08-17 16:20:26 -07:00
Kbuild
Kconfig	kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt	2018-08-02 08:06:55 +09:00
MAINTAINERS	MAINTAINERS: Add Sasha as a stable branch maintainer	2018-12-01 09:37:25 +01:00
Makefile	Linux 4.19.9	2018-12-13 09:16:23 +01:00
README	Docs: Added a pointer to the formatted docs to README	2018-03-21 09:02:53 -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.
See Documentation/00-INDEX for a list of what is contained in each file.

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.