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Jesper Dangaard Brouer 5442a9da69 veth: more robust handing of race to avoid txq getting stuck 
Commit dc82a33297 ("veth: apply qdisc backpressure on full ptr_ring to
reduce TX drops") introduced a race condition that can lead to a permanently
stalled TXQ. This was observed in production on ARM64 systems (Ampere Altra
Max).

The race occurs in veth_xmit(). The producer observes a full ptr_ring and
stops the queue (netif_tx_stop_queue()). The subsequent conditional logic,
intended to re-wake the queue if the consumer had just emptied it (if
(__ptr_ring_empty(...)) netif_tx_wake_queue()), can fail. This leads to a
"lost wakeup" where the TXQ remains stopped (QUEUE_STATE_DRV_XOFF) and
traffic halts.

This failure is caused by an incorrect use of the __ptr_ring_empty() API
from the producer side. As noted in kernel comments, this check is not
guaranteed to be correct if a consumer is operating on another CPU. The
empty test is based on ptr_ring->consumer_head, making it reliable only for
the consumer. Using this check from the producer side is fundamentally racy.

This patch fixes the race by adopting the more robust logic from an earlier
version V4 of the patchset, which always flushed the peer:

(1) In veth_xmit(), the racy conditional wake-up logic and its memory barrier
are removed. Instead, after stopping the queue, we unconditionally call
__veth_xdp_flush(rq). This guarantees that the NAPI consumer is scheduled,
making it solely responsible for re-waking the TXQ.
  This handles the race where veth_poll() consumes all packets and completes
NAPI *before* veth_xmit() on the producer side has called netif_tx_stop_queue.
The __veth_xdp_flush(rq) will observe rx_notify_masked is false and schedule
NAPI.

(2) On the consumer side, the logic for waking the peer TXQ is moved out of
veth_xdp_rcv() and placed at the end of the veth_poll() function. This
placement is part of fixing the race, as the netif_tx_queue_stopped() check
must occur after rx_notify_masked is potentially set to false during NAPI
completion.
  This handles the race where veth_poll() consumes all packets, but haven't
finished (rx_notify_masked is still true). The producer veth_xmit() stops the
TXQ and __veth_xdp_flush(rq) will observe rx_notify_masked is true, meaning
not starting NAPI.  Then veth_poll() change rx_notify_masked to false and
stops NAPI.  Before exiting veth_poll() will observe TXQ is stopped and wake
it up.

Fixes: dc82a33297 ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
Reviewed-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/176295323282.307447.14790015927673763094.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2025-11-14 18:16:53 -08:00
arch LoongArch fixes for v6.18-rc6 2025-11-12 18:21:30 -08:00
block block-6.18-20251031 2025-10-31 12:57:19 -07:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto This push contains the following changes: 2025-10-10 08:56:16 -07:00
Documentation Including fixes from Bluetooth and Wireless. No known outstanding 2025-11-13 11:20:25 -08:00
drivers veth: more robust handing of race to avoid txq getting stuck 2025-11-14 18:16:53 -08:00
fs Changes since last update: 2025-11-13 05:02:59 -08:00
include Including fixes from Bluetooth and Wireless. No known outstanding 2025-11-13 11:20:25 -08:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring: fix regbuf vector size truncation 2025-11-07 17:17:13 -07:00
ipc namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
kernel kho: warn and exit when unpreserved page wasn't preserved 2025-11-09 21:19:47 -08:00
lib maple_tree: fix tracepoint string pointers 2025-11-09 21:19:45 -08:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/secretmem: fix use-after-free race in fault handler 2025-11-09 21:19:46 -08:00
net net: openvswitch: remove never-working support for setting nsh fields 2025-11-14 18:13:24 -08:00
rust rust: Add -fno-isolate-erroneous-paths-dereference to bindgen_skip_c_flags 2025-11-10 08:37:06 +08:00
samples Char/Misc/IIO/Binder changes for 6.18-rc1 2025-10-04 16:26:32 -07:00
scripts scripts/decode_stacktrace.sh: fix build ID and PC source parsing 2025-11-09 21:19:45 -08:00
security integrity-v6.18 2025-10-05 10:48:33 -07:00
sound ASoC: Fixes for v6.18 2025-10-30 13:08:08 +01:00
tools Including fixes from Bluetooth and Wireless. No known outstanding 2025-11-13 11:20:25 -08:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying 2025-11-04 09:16:53 -08:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap KVM/arm654 fixes for 6.18, take #2 2025-11-09 08:07:55 +01:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.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: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS MAINTAINERS: Remove eth bridge website 2025-11-13 16:24:14 -08:00
Makefile Linux 6.18-rc5 2025-11-09 15:10:19 -08:00
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.