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Frederic Weisbecker 6e641d499b timers/migration: Enforce group initialization visibility to tree walkers 
commit de3ced72a7 upstream.

Commit 2522c84db513 ("timers/migration: Fix another race between hotplug
and idle entry/exit") fixed yet another race between idle exit and CPU
hotplug up leading to a wrong "0" value migrator assigned to the top
level. However there is yet another situation that remains unhandled:

         [GRP0:0]
      migrator  = TMIGR_NONE
      active    = NONE
      groupmask = 1
      /     \      \
     0       1     2..7
   idle      idle   idle

0) The system is fully idle.

         [GRP0:0]
      migrator  = CPU 0
      active    = CPU 0
      groupmask = 1
      /     \      \
     0       1     2..7
   active   idle   idle

1) CPU 0 is activating. It has done the cmpxchg on the top's ->migr_state
but it hasn't yet returned to __walk_groups().

         [GRP0:0]
      migrator  = CPU 0
      active    = CPU 0, CPU 1
      groupmask = 1
      /     \      \
     0       1     2..7
   active  active  idle

2) CPU 1 is activating. CPU 0 stays the migrator (still stuck in
__walk_groups(), delayed by #VMEXIT for example).

                    [GRP1:0]
                migrator = TMIGR_NONE
                active   = NONE
                groupmask = 1
             /                   \
         [GRP0:0]                  [GRP0:1]
      migrator  = CPU 0           migrator = TMIGR_NONE
      active    = CPU 0, CPU1     active   = NONE
      groupmask = 1               groupmask = 2
      /     \      \
     0       1     2..7                   8
   active  active  idle                !online

3) CPU 8 is preparing to boot. CPUHP_TMIGR_PREPARE is being ran by CPU 1
which has created the GRP0:1 and the new top GRP1:0 connected to GRP0:1
and GRP0:0. CPU 1 hasn't yet propagated its activation up to GRP1:0.

                    [GRP1:0]
               migrator = GRP0:0
               active   = GRP0:0
               groupmask = 1
             /                   \
         [GRP0:0]                  [GRP0:1]
     migrator  = CPU 0           migrator = TMIGR_NONE
     active    = CPU 0, CPU1     active   = NONE
     groupmask = 1               groupmask = 2
     /     \      \
    0       1     2..7                   8
  active  active  idle                !online

4) CPU 0 finally resumed after its #VMEXIT. It's in __walk_groups()
returning from tmigr_cpu_active(). The new top GRP1:0 is visible and
fetched and the pre-initialized groupmask of GRP0:0 is also visible.
As a result tmigr_active_up() is called to GRP1:0 with GRP0:0 as active
and migrator. CPU 0 is returning to __walk_groups() but suffers again
a #VMEXIT.

                    [GRP1:0]
               migrator = GRP0:0
               active   = GRP0:0
               groupmask = 1
             /                   \
         [GRP0:0]                  [GRP0:1]
     migrator  = CPU 0           migrator = TMIGR_NONE
     active    = CPU 0, CPU1     active   = NONE
     groupmask = 1               groupmask = 2
     /     \      \
    0       1     2..7                   8
  active  active  idle                 !online

5) CPU 1 propagates its activation of GRP0:0 to GRP1:0. This has no
   effect since CPU 0 did it already.

                    [GRP1:0]
               migrator = GRP0:0
               active   = GRP0:0, GRP0:1
               groupmask = 1
             /                   \
         [GRP0:0]                  [GRP0:1]
     migrator  = CPU 0           migrator = CPU 8
     active    = CPU 0, CPU1     active   = CPU 8
     groupmask = 1               groupmask = 2
     /     \      \                     \
    0       1     2..7                   8
  active  active  idle                 active

6) CPU 1 links CPU 8 to its group. CPU 8 boots and goes through
   CPUHP_AP_TMIGR_ONLINE which propagates activation.

                                   [GRP2:0]
                              migrator = TMIGR_NONE
                              active   = NONE
                              groupmask = 1
                             /                \
                    [GRP1:0]                    [GRP1:1]
               migrator = GRP0:0              migrator = TMIGR_NONE
               active   = GRP0:0, GRP0:1      active   = NONE
               groupmask = 1                  groupmask = 2
             /                   \
         [GRP0:0]                  [GRP0:1]                [GRP0:2]
     migrator  = CPU 0           migrator = CPU 8        migrator = TMIGR_NONE
     active    = CPU 0, CPU1     active   = CPU 8        active   = NONE
     groupmask = 1               groupmask = 2           groupmask = 0
     /     \      \                     \
    0       1     2..7                   8                  64
  active  active  idle                 active             !online

7) CPU 64 is booting. CPUHP_TMIGR_PREPARE is being ran by CPU 1
which has created the GRP1:1, GRP0:2 and the new top GRP2:0 connected to
GRP1:1 and GRP1:0. CPU 1 hasn't yet propagated its activation up to
GRP2:0.

                                   [GRP2:0]
                              migrator = 0 (!!!)
                              active   = NONE
                              groupmask = 1
                             /                \
                    [GRP1:0]                    [GRP1:1]
               migrator = GRP0:0              migrator = TMIGR_NONE
               active   = GRP0:0, GRP0:1      active   = NONE
               groupmask = 1                  groupmask = 2
             /                   \
         [GRP0:0]                  [GRP0:1]                [GRP0:2]
     migrator  = CPU 0           migrator = CPU 8        migrator = TMIGR_NONE
     active    = CPU 0, CPU1     active   = CPU 8        active   = NONE
     groupmask = 1               groupmask = 2           groupmask = 0
     /     \      \                     \
    0       1     2..7                   8                  64
  active  active  idle                 active             !online

8) CPU 0 finally resumed after its #VMEXIT. It's in __walk_groups()
returning from tmigr_cpu_active(). The new top GRP2:0 is visible and
fetched but the pre-initialized groupmask of GRP1:0 is not because no
ordering made its initialization visible. As a result tmigr_active_up()
may be called to GRP2:0 with a "0" child's groumask. Leaving the timers
ignored for ever when the system is fully idle.

The race is highly theoretical and perhaps impossible in practice but
the groupmask of the child is not the only concern here as the whole
initialization of the child is not guaranteed to be visible to any
tree walker racing against hotplug (idle entry/exit, remote handling,
etc...). Although the current code layout seem to be resilient to such
hazards, this doesn't tell much about the future.

Fix this with enforcing address dependency between group initialization
and the write/read to the group's parent's pointer. Fortunately that
doesn't involve any barrier addition in the fast paths.

Fixes: 10a0e6f3d3 ("timers/migration: Move hierarchy setup into cpuhotplug prepare callback")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250114231507.21672-3-frederic@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-01-23 17:23:03 +01:00
arch x86/asm: Make serialize() always_inline 2025-01-23 17:22:58 +01:00
block block, bfq: fix waker_bfqq UAF after bfq_split_bfqq() 2025-01-17 13:40:59 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: ecdsa - Avoid signed integer overflow on signature decoding 2024-12-14 20:03:36 +01:00
Documentation fs/writeback: convert wbc_account_cgroup_owner to take a folio 2025-01-17 13:40:33 +01:00
drivers irqchip/gic-v3-its: Don't enable interrupts in its_irq_set_vcpu_affinity() 2025-01-23 17:23:02 +01:00
fs fs/proc: fix softlockup in __read_vmcore (part 2) 2025-01-23 17:23:01 +01:00
include hrtimers: Handle CPU state correctly on hotplug 2025-01-23 17:23:03 +01:00
init Compiler Attributes: disable __counted_by for clang < 19.1.3 2024-12-05 14:02:45 +01:00
io_uring io_uring: don't touch sqd->thread off tw add 2025-01-17 13:40:59 +01:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-05 14:03:02 +01:00
kernel timers/migration: Enforce group initialization visibility to tree walkers 2025-01-23 17:23:03 +01:00
lib maple_tree: reload mas before the second call for mas_empty_area 2025-01-09 13:33:50 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm: vmscan : pgdemote vmstat is not getting updated when MGLRU is enabled. 2025-01-23 17:23:02 +01:00
net net/ncsi: fix locking in Get MAC Address handling 2025-01-23 17:23:00 +01:00
rust rust: kbuild: set bindgen's Rust target version 2024-12-19 18:13:22 +01:00
samples samples/bpf: Fix a resource leak 2024-12-14 20:03:46 +01:00
scripts scripts/sorttable: fix orc_sort_cmp() to maintain symmetry and transitivity 2025-01-09 13:33:50 +01:00
security selinux: ignore unknown extended permissions 2025-01-09 13:33:32 +01:00
sound ALSA: hda/realtek: fixup ASUS H7606W 2025-01-23 17:22:58 +01:00
tools selftests/mm: set allocated memory to non-zero content in cow test 2025-01-23 17:23:01 +01:00
usr initramfs: shorten cmd_initfs in usr/Makefile 2024-07-16 01:07:52 +09:00
virt KVM: x86: switch hugepage recovery thread to vhost_task 2024-12-05 14:02:43 +01:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore Kbuild updates for v6.12 2024-09-24 13:02:06 -07:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: update location of media main tree 2024-12-05 14:02:42 +01:00
Makefile Linux 6.12.10 2025-01-17 13:41:00 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -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 reStructuredText 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.