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Alexander Lobakin 0315a075f1 net: fix premature exit from NAPI state polling in napi_disable() 
Commit 719c571970 ("net: make napi_disable() symmetric with
enable") accidentally introduced a bug sometimes leading to a kernel
BUG when bringing an iface up/down under heavy traffic load.

Prior to this commit, napi_disable() was polling n->state until
none of (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC) is set and then
always flip them. Now there's a possibility to get away with the
NAPIF_STATE_SCHE unset as 'continue' drops us to the cmpxchg()
call with an uninitialized variable, rather than straight to
another round of the state check.

Error path looks like:

napi_disable():
unsigned long val, new; /* new is uninitialized */

do {
	val = READ_ONCE(n->state); /* NAPIF_STATE_NPSVC and/or
				      NAPIF_STATE_SCHED is set */
	if (val & (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC)) { /* true */
		usleep_range(20, 200);
		continue; /* go straight to the condition check */
	}
	new = val | <...>
} while (cmpxchg(&n->state, val, new) != val); /* state == val, cmpxchg()
						  writes garbage */

napi_enable():
do {
	val = READ_ONCE(n->state);
	BUG_ON(!test_bit(NAPI_STATE_SCHED, &val)); /* 50/50 boom */
<...>

while the typical BUG splat is like:

[  172.652461] ------------[ cut here ]------------
[  172.652462] kernel BUG at net/core/dev.c:6937!
[  172.656914] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[  172.661966] CPU: 36 PID: 2829 Comm: xdp_redirect_cp Tainted: G          I       5.15.0 #42
[  172.670222] Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.02.01.0014.082620210524 08/26/2021
[  172.680646] RIP: 0010:napi_enable+0x5a/0xd0
[  172.684832] Code: 07 49 81 cc 00 01 00 00 4c 89 e2 48 89 d8 80 e6 fb f0 48 0f b1 55 10 48 39 c3 74 10 48 8b 5d 10 f6 c7 04 75 3d f6 c3 01 75 b4 <0f> 0b 5b 5d 41 5c c3 65 ff 05 b8 e5 61 53 48 c7 c6 c0 f3 34 ad 48
[  172.703578] RSP: 0018:ffffa3c9497477a8 EFLAGS: 00010246
[  172.708803] RAX: ffffa3c96615a014 RBX: 0000000000000000 RCX: ffff8a4b575301a0
< snip >
[  172.782403] Call Trace:
[  172.784857]  <TASK>
[  172.786963]  ice_up_complete+0x6f/0x210 [ice]
[  172.791349]  ice_xdp+0x136/0x320 [ice]
[  172.795108]  ? ice_change_mtu+0x180/0x180 [ice]
[  172.799648]  dev_xdp_install+0x61/0xe0
[  172.803401]  dev_xdp_attach+0x1e0/0x550
[  172.807240]  dev_change_xdp_fd+0x1e6/0x220
[  172.811338]  do_setlink+0xee8/0x1010
[  172.814917]  rtnl_setlink+0xe5/0x170
[  172.818499]  ? bpf_lsm_binder_set_context_mgr+0x10/0x10
[  172.823732]  ? security_capable+0x36/0x50
< snip >

Fix this by replacing 'do { } while (cmpxchg())' with an "infinite"
for-loop with an explicit break.

From v1 [0]:
 - just use a for-loop to simplify both the fix and the existing
   code (Eric).

[0] https://lore.kernel.org/netdev/20211110191126.1214-1-alexandr.lobakin@intel.com

Fixes: 719c571970 ("net: make napi_disable() symmetric with enable")
Suggested-by: Eric Dumazet <edumazet@google.com> # for-loop
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211110195605.1304-1-alexandr.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2021-11-10 17:45:15 -08:00
arch riscv, bpf: Fix RV32 broken build, and silence RV64 warning 2021-11-05 16:52:34 +01:00
block for-5.16/inode-sync-2021-10-29 2021-11-01 10:25:27 -07:00
certs certs: Add support for using elliptic curve keys for signing modules 2021-08-23 19:55:42 +03:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-11-01 21:24:02 -07:00
Documentation net: udp: correct the document for udp_mem 2021-11-05 10:42:46 +00:00
drivers net/mlx5: Lag, fix a potential Oops with mlx5_lag_create_definer() 2021-11-10 14:43:20 +00:00
fs libfs: Support RENAME_EXCHANGE in simple_rename() 2021-11-03 15:43:08 +01:00
include Merge https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf 2021-11-09 15:44:48 -08:00
init Tracing updates for 5.16: 2021-11-01 20:05:19 -07:00
ipc ipc: remove memcg accounting for sops objects in do_semtimedop() 2021-09-14 10:22:11 -07:00
kernel bpf: Stop caching subprog index in the bpf_pseudo_func insn 2021-11-06 12:54:12 -07:00
lib Core: 2021-11-02 06:20:58 -07:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm libfs: Move shmem_exchange to simple_rename_exchange 2021-11-03 15:43:00 +01:00
net net: fix premature exit from NAPI state polling in napi_disable() 2021-11-10 17:45:15 -08:00
samples Core: 2021-11-02 06:20:58 -07:00
scripts Core: 2021-11-02 06:20:58 -07:00
security security: implement sctp_assoc_established hook in selinux 2021-11-03 11:09:20 +00:00
sound ALSA: usb-audio: Fix microphone sound on Jieli webcam. 2021-10-19 08:07:01 +02:00
tools selftests: forwarding: Fix packet matching in mirroring selftests 2021-11-10 14:38:44 +00:00
usr .gitignore: prefix local generated files with a slash 2021-05-02 00:43:35 +09:00
virt KVM: Remove tlbs_dirty 2021-09-23 11:01:12 -04:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig
.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: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap media updates for v5.16-rc1 2021-11-01 18:45:08 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
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 ENA maintainers information 2021-11-02 19:15:46 -07:00
Makefile Core: 2021-11-02 06:20:58 -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.