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Nicholas Piggin bafb056ce2 sparc64: remove mm_cpumask clearing to fix kthread_use_mm race The de facto (and apparently uncommented) standard for using an mm had, thanks to this code in sparc if nothing else, been that you must have a reference on mm_users and that reference must have been obtained with mmget(), i.e., from a thread with a reference to mm_users that had used the mm. The introduction of mmget_not_zero() in commit `d2005e3f41` ("userfaultfd: don't pin the user memory in userfaultfd_file_create()") allowed mm_count holders to aoperate on user mappings asynchronously from the actual threads using the mm, but they were not to load those mappings into their TLB (i.e., walking vmas and page tables is okay, kthread_use_mm() is not). io_uring `2b188cc1bb` ("Add io_uring IO interface") added code which does a kthread_use_mm() from a mmget_not_zero() refcount. The problem with this is code which previously assumed mm == current->mm and mm->mm_users == 1 implies the mm will remain single-threaded at least until this thread creates another mm_users reference, has now broken. arch/sparc/kernel/smp_64.c: if (atomic_read(&mm->mm_users) == 1) { cpumask_copy(mm_cpumask(mm), cpumask_of(cpu)); goto local_flush_and_out; } vs fs/io_uring.c if (unlikely(!(ctx->flags & IORING_SETUP_SQPOLL) \|\| !mmget_not_zero(ctx->sqo_mm))) return -EFAULT; kthread_use_mm(ctx->sqo_mm); mmget_not_zero() could come in right after the mm_users == 1 test, then kthread_use_mm() which sets its CPU in the mm_cpumask. That update could be lost if cpumask_copy() occurs afterward. I propose we fix this by allowing mmget_not_zero() to be a first-class reference, and not have this obscure undocumented and unchecked restriction. The basic fix for sparc64 is to remove its mm_cpumask clearing code. The optimisation could be effectively restored by sending IPIs to mm_cpumask members and having them remove themselves from mm_cpumask. This is more tricky so I leave it as an exercise for someone with a sparc64 SMP. powerpc has a (currently similarly broken) example. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Acked-by: David S. Miller <davem@davemloft.net> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20200914045219.3736466-4-npiggin@gmail.com		History 2020-09-16 12:24:37 +10:00
arch	sparc64: remove mm_cpumask clearing to fix kthread_use_mm race	2020-09-16 12:24:37 +10:00
block	block-5.9-2020-08-14	2020-08-15 20:36:42 -07:00
certs	.gitignore: add SPDX License Identifier	2020-03-25 11:50:48 +01:00
crypto	Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6	2020-08-14 13:09:15 -07:00
Documentation	powerpc fixes for 5.9 #3	2020-08-23 11:37:23 -07:00
drivers	A set of EFI fixes:	2020-08-23 11:08:32 -07:00
fs	mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race	2020-09-16 12:24:31 +10:00
include	Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net	2020-08-23 10:52:33 -07:00
init	OpenRISC updates for 5.9	2020-08-14 14:04:53 -07:00
ipc	ipc/shm.c: remove the superfluous break	2020-08-12 10:58:02 -07:00
kernel	A single bug fix for the common entry code. The transcript of the x86	2020-08-23 11:05:47 -07:00
lib	lib/string.c: Use freestanding environment	2020-08-19 11:23:45 -07:00
LICENSES	LICENSES: Rename other to deprecated	2019-05-03 06:34:32 -06:00
mm	mm, page_alloc: fix core hung in free_pcppages_bulk()	2020-08-21 09:52:53 -07:00
net	net: nexthop: don't allow empty NHA_GROUP	2020-08-22 12:39:55 -07:00
samples	Kbuild updates for v5.9	2020-08-09 14:10:26 -07:00
scripts	kconfig: qconf: replace deprecated QString::sprintf() with QTextStream	2020-08-21 10:23:38 +09:00
security	Merge branch 'akpm' (patches from Andrew)	2020-08-12 11:24:12 -07:00
sound	sound fixes for 5.9-rc2	2020-08-21 10:07:54 -07:00
tools	Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net	2020-08-23 10:52:33 -07:00
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	* PAE and PKU bugfixes for x86	2020-08-22 10:03:05 -07:00
.clang-format	block: add bio_for_each_bvec_all()	2020-05-25 11:25:24 +02:00
.cocciconfig	scripts: add Linux .cocciconfig for coccinelle	2016-07-22 12:13:39 +02:00
.get_maintainer.ignore	Opt out of scripts/get_maintainer.pl	2019-05-16 10:53:40 -07:00
.gitattributes	.gitattributes: use 'dts' diff driver for dts files	2019-12-04 19:44:11 -08:00
.gitignore	.gitignore: Add ZSTD-compressed files	2020-07-31 11:50:49 +02:00
.mailmap	Merge branch 'akpm' (patches from Andrew)	2020-08-21 14:44:48 -07:00
COPYING	COPYING: state that all contributions really are covered by this file	2020-02-10 13:32:20 -08:00
CREDITS	CREDITS: Replace HTTP links with HTTPS ones	2020-07-23 14:53:58 -06:00
Kbuild	kbuild: rename hostprogs-y/always to hostprogs/always-y	2020-02-04 01:53:07 +09:00
Kconfig	kbuild: ensure full rebuild when the compiler is updated	2020-05-12 13:28:33 +09:00
MAINTAINERS	MAINTAINERS: Update Mellanox and Cumulus Network addresses to new domain	2020-08-21 10:48:48 -03:00
Makefile	Linux 5.9-rc2	2020-08-23 14:08:43 -07:00
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.