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Vladimir Oltean 11e25a2b5d ptp: fix breakage after ptp_vclock_in_use() rework 
[ Upstream commit 5ab73b010c ]

What is broken
--------------

ptp4l, and any other application which calls clock_adjtime() on a
physical clock, is greeted with error -EBUSY after commit 87f7ce260a
("ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()").

Explanation for the breakage
----------------------------

The blamed commit was based on the false assumption that
ptp_vclock_in_use() callers already test for n_vclocks prior to calling
this function.

This is notably incorrect for the code path below, in which there is, in
fact, no n_vclocks test:

ptp_clock_adjtime()
-> ptp_clock_freerun()
   -> ptp_vclock_in_use()

The result is that any clock adjustment on any physical clock is now
impossible. This is _despite_ there not being any vclock over this
physical clock.

$ ptp4l -i eno0 -2 -P -m
ptp4l[58.425]: selected /dev/ptp0 as PTP clock
[   58.429749] ptp: physical clock is free running
ptp4l[58.431]: Failed to open /dev/ptp0: Device or resource busy
failed to create a clock
$ cat /sys/class/ptp/ptp0/n_vclocks
0

The patch makes the ptp_vclock_in_use() function say "if it's not a
virtual clock, then this physical clock does have virtual clocks on
top".

Then ptp_clock_freerun() uses this information to say "this physical
clock has virtual clocks on top, so it must stay free-running".

Then ptp_clock_adjtime() uses this information to say "well, if this
physical clock has to be free-running, I can't do it, return -EBUSY".

Simply put, ptp_vclock_in_use() cannot be simplified so as to remove the
test whether vclocks are in use.

What did the blamed commit intend to fix
----------------------------------------

The blamed commit presents a lockdep warning stating "possible recursive
locking detected", with the n_vclocks_store() and ptp_clock_unregister()
functions involved.

The recursive locking seems this:
n_vclocks_store()
-> mutex_lock_interruptible(&ptp->n_vclocks_mux) // 1
-> device_for_each_child_reverse(..., unregister_vclock)
   -> unregister_vclock()
      -> ptp_vclock_unregister()
         -> ptp_clock_unregister()
            -> ptp_vclock_in_use()
               -> mutex_lock_interruptible(&ptp->n_vclocks_mux) // 2

The issue can be triggered by creating and then deleting vclocks:
$ echo 2 > /sys/class/ptp/ptp0/n_vclocks
$ echo 0 > /sys/class/ptp/ptp0/n_vclocks

But note that in the original stack trace, the address of the first lock
is different from the address of the second lock. This is because at
step 1 marked above, &ptp->n_vclocks_mux is the lock of the parent
(physical) PTP clock, and at step 2, the lock is of the child (virtual)
PTP clock. They are different locks of different devices.

In this situation there is no real deadlock, the lockdep warning is
caused by the fact that the mutexes have the same lock class on both the
parent and the child. Functionally it is fine.

Proposed alternative solution
-----------------------------

We must reintroduce the body of ptp_vclock_in_use() mostly as it was
structured prior to the blamed commit, but avoid the lockdep warning.

Based on the fact that vclocks cannot be nested on top of one another
(ptp_is_attribute_visible() hides n_vclocks for virtual clocks), we
already know that ptp->n_vclocks is zero for a virtual clock. And
ptp->is_virtual_clock is a runtime invariant, established at
ptp_clock_register() time and never changed. There is no need to
serialize on any mutex in order to read ptp->is_virtual_clock, and we
take advantage of that by moving it outside the lock.

Thus, virtual clocks do not need to acquire &ptp->n_vclocks_mux at
all, and step 2 in the code walkthrough above can simply go away.
We can simply return false to the question "ptp_vclock_in_use(a virtual
clock)".

Other notes
-----------

Releasing &ptp->n_vclocks_mux before ptp_vclock_in_use() returns
execution seems racy, because the returned value can become stale as
soon as the function returns and before the return value is used (i.e.
n_vclocks_store() can run any time). The locking requirement should
somehow be transferred to the caller, to ensure a longer life time for
the returned value, but this seems out of scope for this severe bug fix.

Because we are also fixing up the logic from the original commit, there
is another Fixes: tag for that.

Fixes: 87f7ce260a ("ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()")
Fixes: 73f37068d5 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250613174749.406826-2-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-06-27 11:11:43 +01:00
arch x86/tools: Drop duplicate unlikely() definition in insn_decoder_test.c 2025-06-27 11:11:41 +01:00
block block: Clear BIO_EMULATES_ZONE_APPEND flag on BIO completion 2025-06-27 11:11:19 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: api - Redo lookup on EEXIST 2025-06-19 15:31:30 +02:00
Documentation kbuild: rust: add rustc-min-version support function 2025-06-27 11:11:40 +01:00
drivers ptp: fix breakage after ptp_vclock_in_use() rework 2025-06-27 11:11:43 +01:00
fs ksmbd: add free_transport ops in ksmbd connection 2025-06-27 11:11:42 +01:00
include ipv6: replace ipcm6_init calls with ipcm6_init_sk 2025-06-27 11:11:41 +01:00
init rust: clean Rust 1.88.0's unnecessary_transmutes lint 2025-05-18 08:24:51 +02:00
io_uring io_uring/sqpoll: don't put task_struct on tctx setup failure 2025-06-27 11:11:42 +01:00
ipc ipc: fix to protect IPCS lookups using RCU 2025-06-27 11:11:22 +01:00
kernel workqueue: Initialize wq_isolated_cpumask in workqueue_init_early() 2025-06-27 11:11:42 +01:00
lib pldmfw: Select CRC32 when PLDMFW is selected 2025-06-27 11:11:42 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm/hugetlb: unshare page tables during VMA split, not before 2025-06-27 11:11:40 +01:00
net net: netmem: fix skb_ensure_writable with unreadable skbs 2025-06-27 11:11:43 +01:00
rust rust: compile libcore with edition 2024 for 1.87+ 2025-06-27 11:11:40 +01:00
samples samples/bpf: Fix compilation failure for samples/bpf on LoongArch Fedora 2025-05-29 11:02:01 +02:00
scripts rust: compile libcore with edition 2024 for 1.87+ 2025-06-27 11:11:40 +01:00
security selinux: fix selinux_xfrm_alloc_user() to set correct ctx_len 2025-06-27 11:11:38 +01:00
sound ALSA: hda/realtek: Add quirk for Asus GU605C 2025-06-27 11:11:40 +01:00
tools bpf: Fix L4 csum update on IPv6 in CHECKSUM_COMPLETE 2025-06-27 11:11:40 +01:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-13 13:02:18 +01:00
virt KVM: Add member to struct kvm_gfn_range to indicate private/shared 2025-05-22 14:29:36 +02:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-18 08:24:51 +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 rust: introduce .clippy.toml 2025-03-13 13:01:42 +01:00
.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 Alexey Makhalov's email address 2025-05-22 14:29:46 +02:00
Makefile Linux 6.12.34 2025-06-19 15:32:38 +02:00
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.