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David Howells 32eff2f22b rxrpc: Fix ack discard 
[ Upstream commit 441fdee1ea ]

The Rx protocol has a "previousPacket" field in it that is not handled in
the same way by all protocol implementations.  Sometimes it contains the
serial number of the last DATA packet received, sometimes the sequence
number of the last DATA packet received and sometimes the highest sequence
number so far received.

AF_RXRPC is using this to weed out ACKs that are out of date (it's possible
for ACK packets to get reordered on the wire), but this does not work with
OpenAFS which will just stick the sequence number of the last packet seen
into previousPacket.

The issue being seen is that big AFS FS.StoreData RPC (eg. of ~256MiB) are
timing out when partly sent.  A trace was captured, with an additional
tracepoint to show ACKs being discarded in rxrpc_input_ack().  Here's an
excerpt showing the problem.

 52873.203230: rxrpc_tx_data: c=000004ae DATA ed1a3584:00000002 0002449c q=00024499 fl=09

A DATA packet with sequence number 00024499 has been transmitted (the "q="
field).

 ...
 52873.243296: rxrpc_rx_ack: c=000004ae 00012a2b DLY r=00024499 f=00024497 p=00024496 n=0
 52873.243376: rxrpc_rx_ack: c=000004ae 00012a2c IDL r=0002449b f=00024499 p=00024498 n=0
 52873.243383: rxrpc_rx_ack: c=000004ae 00012a2d OOS r=0002449d f=00024499 p=0002449a n=2

The Out-Of-Sequence ACK indicates that the server didn't see DATA sequence
number 00024499, but did see seq 0002449a (previousPacket, shown as "p=",
skipped the number, but firstPacket, "f=", which shows the bottom of the
window is set at that point).

 52873.252663: rxrpc_retransmit: c=000004ae q=24499 a=02 xp=14581537
 52873.252664: rxrpc_tx_data: c=000004ae DATA ed1a3584:00000002 000244bc q=00024499 fl=0b *RETRANS*

The packet has been retransmitted.  Retransmission recurs until the peer
says it got the packet.

 52873.271013: rxrpc_rx_ack: c=000004ae 00012a31 OOS r=000244a1 f=00024499 p=0002449e n=6

More OOS ACKs indicate that the other packets that are already in the
transmission pipeline are being received.  The specific-ACK list is up to 6
ACKs and NAKs.

 ...
 52873.284792: rxrpc_rx_ack: c=000004ae 00012a49 OOS r=000244b9 f=00024499 p=000244b6 n=30
 52873.284802: rxrpc_retransmit: c=000004ae q=24499 a=0a xp=63505500
 52873.284804: rxrpc_tx_data: c=000004ae DATA ed1a3584:00000002 000244c2 q=00024499 fl=0b *RETRANS*
 52873.287468: rxrpc_rx_ack: c=000004ae 00012a4a OOS r=000244ba f=00024499 p=000244b7 n=31
 52873.287478: rxrpc_rx_ack: c=000004ae 00012a4b OOS r=000244bb f=00024499 p=000244b8 n=32

At this point, the server's receive window is full (n=32) with presumably 1
NAK'd packet and 31 ACK'd packets.  We can't transmit any more packets.

 52873.287488: rxrpc_retransmit: c=000004ae q=24499 a=0a xp=61327980
 52873.287489: rxrpc_tx_data: c=000004ae DATA ed1a3584:00000002 000244c3 q=00024499 fl=0b *RETRANS*
 52873.293850: rxrpc_rx_ack: c=000004ae 00012a4c DLY r=000244bc f=000244a0 p=00024499 n=25

And now we've received an ACK indicating that a DATA retransmission was
received.  7 packets have been processed (the occupied part of the window
moved, as indicated by f= and n=).

 52873.293853: rxrpc_rx_discard_ack: c=000004ae r=00012a4c 000244a0<00024499 00024499<000244b8

However, the DLY ACK gets discarded because its previousPacket has gone
backwards (from p=000244b8, in the ACK at 52873.287478 to p=00024499 in the
ACK at 52873.293850).

We then end up in a continuous cycle of retransmit/discard.  kafs fails to
update its window because it's discarding the ACKs and can't transmit an
extra packet that would clear the issue because the window is full.
OpenAFS doesn't change the previousPacket value in the ACKs because no new
DATA packets are received with a different previousPacket number.

Fix this by altering the discard check to only discard an ACK based on
previousPacket if there was no advance in the firstPacket.  This allows us
to transmit a new packet which will cause previousPacket to advance in the
next ACK.

The check, however, needs to allow for the possibility that previousPacket
may actually have had the serial number placed in it instead - in which
case it will go outside the window and we should ignore it.

Fixes: 1a2391c30c ("rxrpc: Fix detection of out of order acks")
Reported-by: Dave Botsch <botsch@cnf.cornell.edu>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2020-05-27 17:37:46 +02:00
arch x86/unwind/orc: Fix unwind_get_return_address_ptr() for inactive tasks 2020-05-27 17:37:44 +02:00
block block, bfq: fix use-after-free in bfq_idle_slice_timer_body 2020-04-17 10:48:42 +02:00
certs export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR() 2018-08-22 23:21:44 +09:00
crypto gcc-10: avoid shadowing standard library 'free()' in crypto 2020-05-20 08:18:46 +02:00
Documentation arm64: errata: Hide CTR_EL0.DIC on systems affected by Neoverse-N1 #1542419 2020-04-29 16:31:08 +02:00
drivers iio: adc: stm32-dfsdm: fix device used to request dma 2020-05-27 17:37:45 +02:00
firmware Fix built-in early-load Intel microcode alignment 2020-01-23 08:21:29 +01:00
fs Revert "gfs2: Don't demote a glock until its revokes are written" 2020-05-27 17:37:42 +02:00
include rxrpc: Trace discarded ACKs 2020-05-27 17:37:46 +02:00
init x86: Fix early boot crash on gcc-10, third try 2020-05-20 08:18:49 +02:00
ipc ipc/util.c: sysvipc_find_ipc() incorrectly updates position index 2020-05-20 08:18:40 +02:00
kernel padata: purge get_cpu and reorder_via_wq from padata_do_serial 2020-05-27 17:37:36 +02:00
lib ubsan: build ubsan.c more conservatively 2020-05-27 17:37:27 +02:00
LICENSES LICENSES: Remove CC-BY-SA-4.0 license text 2018-10-18 11:28:50 +02:00
mm shmem: fix possible deadlocks on shmlock_user_lock 2020-05-20 08:18:32 +02:00
net rxrpc: Fix ack discard 2020-05-27 17:37:46 +02:00
samples vmalloc: fix remap_vmalloc_range() bounds checks 2020-04-29 16:31:27 +02:00
scripts gcc-common.h: Update for GCC 10 2020-05-27 17:37:30 +02:00
security apparmor: Fix aa_label refcnt leak in policy_update 2020-05-27 17:37:37 +02:00
sound ALSA: hda/realtek - Add more fixup entries for Clevo machines 2020-05-27 17:37:37 +02:00
tools objtool: Fix stack offset tracking for indirect CFAs 2020-05-14 07:57:23 +02:00
usr initramfs: restore default compression behavior 2020-04-13 10:44:59 +02:00
virt KVM: arm64: Fix 32bit PC wrap-around 2020-05-14 07:57:20 +02:00
.clang-format clang-format: Set IndentWrappedFunctionNames false 2018-08-01 18:38:51 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap libnvdimm-for-4.19_misc 2018-08-25 18:13:10 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS 9p: remove Ron Minnich from MAINTAINERS 2018-08-17 16:20:26 -07:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 16:38:49 +01:00
Makefile Linux 4.19.124 2020-05-20 08:18:54 +02:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -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.
See Documentation/00-INDEX for a list of what is contained in each file.

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.