Greeting,

FYI, we noticed the following commit (built with gcc-9):

commit: 18a2dc6982e1aa909b825f0c282dcaa4d53a7d94 ("[PATCH v4 4/4] printk: use the lockless ringbuffer")
url: https://github.com/0day-ci/linux/commits/John-Ogness/printk-replace-ringbuffer/20200707-230114
base: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git 9ebcfadb0610322ac537dd7aa5d9cbc2b2894c68

in testcase: ltp
with following parameters:

	test: kernel_misc

test-description: The LTP testsuite contains a collection of tools for testing the Linux kernel and related features.
test-url: http://linux-test-project.github.io/


on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G

caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):




If you fix the issue, kindly add following tag
Reported-by: kernel test robot <rong.a.chen@intel.com>




<<<test_start>>>
tag=kmsg01 stime=1594295570
cmdline="kmsg01"
contacts=""
analysis=exit
<<<test_output>>>
tst_test.c:1247: INFO: Timeout per run is 0h 05m 00s
kmsg01.c:282: INFO: TEST: injected messages appear in /dev/kmsg
kmsg01.c:324: PASS: injected messages found in log
kmsg01.c:325: PASS: sequence numbers grow as expected
kmsg01.c:232: INFO: TEST: nonblock read
kmsg01.c:237: PASS: non-block read returned EAGAIN
kmsg01.c:248: INFO: TEST: blocking read
kmsg01.c:253: PASS: read blocked
kmsg01.c:264: INFO: TEST: partial read
kmsg01.c:269: PASS: read failed as expected: EINVAL (22)
kmsg01.c:339: INFO: TEST: mult. readers will get same first message
kmsg01.c:375: PASS: all readers got same message on first read
kmsg01.c:393: INFO: TEST: read returns EPIPE when messages get overwritten
kmsg01.c:398: INFO: first seqno: 0
kmsg01.c:411: INFO: first seqno now: 881
kmsg01.c:425: FAIL: read returned: 77: SUCCESS (0)
kmsg01.c:429: INFO: TEST: Subsequent reads() will return available records again
kmsg01.c:433: PASS: after EPIPE read returned next record
kmsg01.c:465: INFO: TEST: seek SEEK_SET 0
kmsg01.c:514: PASS: SEEK_SET 0
kmsg01.c:520: INFO: TEST: seek SEEK_END 0
kmsg01.c:524: PASS: SEEK_END 0
kmsg01.c:530: INFO: TEST: seek SEEK_DATA 0
kmsg01.c:538: PASS: SEEK_DATA 0

Summary:
passed   10
failed   1
skipped  0
warnings 0
<<<execution_status>>>
initiation_status="ok"
duration=46 termination_type=exited termination_id=1 corefile=no
cutime=20 cstime=2085
<<<test_end>>>

       ###############################################################

            Done executing testcases.
            LTP Version:  20200515-84-gc5311d26f
       ###############################################################




To reproduce:

        # build kernel
	cd linux
	cp config-5.8.0-rc3-00004-g18a2dc6982e1a .config
	make HOSTCC=gcc-9 CC=gcc-9 ARCH=x86_64 olddefconfig prepare modules_prepare bzImage modules
	make HOSTCC=gcc-9 CC=gcc-9 ARCH=x86_64 INSTALL_MOD_PATH=<mod-install-dir> modules_install
	cd <mod-install-dir>
	find lib/ | cpio -o -H newc --quiet | gzip > modules.cgz


        git clone https://github.com/intel/lkp-tests.git
        cd lkp-tests
        bin/lkp qemu -k <bzImage> -m modules.cgz job-script # job-script is attached in this email



Thanks,
Rong Chen