Re: [PATCH v2 0/2] printk_ringbuffer: don't needlessly wrap data blocks around

[Petr Mladek <pmladek@suse.com>]

On Thu 2025-09-11 18:18:32, John Ogness wrote:
> On 2025-09-11, Petr Mladek <pmladek@suse.com> wrote:
> > diff --git a/kernel/printk/printk_ringbuffer_kunit_test.c b/kernel/printk/printk_ringbuffer_kunit_test.c
> > index 2282348e869a..241f7ef49ac6 100644
> > --- a/kernel/printk/printk_ringbuffer_kunit_test.c
> > +++ b/kernel/printk/printk_ringbuffer_kunit_test.c
> > @@ -56,7 +56,7 @@ struct prbtest_rbdata {
> >  	char text[] __counted_by(size);
> >  };
> >  
> > -#define MAX_RBDATA_TEXT_SIZE 0x80
> > +#define MAX_RBDATA_TEXT_SIZE (0x256 - sizeof(struct prbtest_rbdata))
> 
> I guess this should be:
> 
> #define MAX_RBDATA_TEXT_SIZE (256  - sizeof(struct prbtest_rbdata))

Great catch!

But the KUnit test fails even with this change, see below. And I am
not surprised. The test should work even with larger-than-allowed
messages. prbtest_writer() should skip then because prb_reserve()
should fail.

Here is test result with:

#define MAX_RBDATA_TEXT_SIZE (256 - sizeof(struct prbtest_rbdata))
#define MAX_PRB_RECORD_SIZE (sizeof(struct prbtest_rbdata) + MAX_RBDATA_TEXT_SIZE)

DEFINE_PRINTKRB(test_rb, 4, 4);

and with this patchset reverted, aka, sources from
printk/linux.git, branch for-next:

[  229.020133] printk_ringbuffer_kunit_test: module verification failed: signature and/or required key missing - tainting kernel
[  229.029453] KTAP version 1
[  229.029793] 1..1
[  229.030461]     KTAP version 1
[  229.030771]     # Subtest: printk-ringbuffer
[  229.031111]     # module: printk_ringbuffer_kunit_test
[  229.031142]     1..1
[  229.032371]     # test_readerwriter: running for 10000 ms
[  229.034348]     # test_readerwriter: start thread 001 (writer)
[  229.034721]     # test_readerwriter: start thread 002 (writer)
[  229.035398]     # test_readerwriter: start thread 003 (writer)
[  229.035697]     # test_readerwriter: start thread 004 (writer)
[  229.038006]     # test_readerwriter: start thread 005 (writer)
[  229.041965]     # test_readerwriter: start thread 006 (writer)
[  229.042244]     # test_readerwriter: start thread 007 (writer)
[  229.042559]     # test_readerwriter: start thread 008 (writer)
[  229.042748]     # test_readerwriter: start thread 009 (writer)
[  229.042996]     # test_readerwriter: start thread 010 (writer)
[  229.043288]     # test_readerwriter: starting test
[  229.043299]     # test_readerwriter: start thread 011 (writer)
[  229.043370]     # test_readerwriter: start reader
[  229.043574]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=15 size=1111638594 text=
[  239.133686]     # test_readerwriter: end reader: read=14 seq=24
[  239.134417]     # test_readerwriter: completed test
[  239.136019]     # test_readerwriter: end thread 011: wrote=10466402
[  239.136856]     # test_readerwriter: end thread 010: wrote=10461343
[  239.137825]     # test_readerwriter: end thread 009: wrote=13686189
[  239.138478]     # test_readerwriter: end thread 008: wrote=10475551
[  239.139106]     # test_readerwriter: end thread 007: wrote=13652622
[  239.139765]     # test_readerwriter: end thread 006: wrote=10473282
[  239.140376]     # test_readerwriter: end thread 005: wrote=10444451
[  239.140989]     # test_readerwriter: end thread 004: wrote=10466857
[  239.141652]     # test_readerwriter: end thread 003: wrote=13733553
[  239.142243]     # test_readerwriter: end thread 002: wrote=13710538
[  239.142859]     # test_readerwriter: end thread 001: wrote=13738411
[  239.143738]     # test_readerwriter.speed: slow
[  239.143771]     not ok 1 test_readerwriter
[  239.144161] not ok 1 printk-ringbuffer

It is well reproducible. It always fails after reading few records.
Here are results from few other runs:

[ 1617.607865]     KTAP version 1
[ 1617.608377]     # Subtest: printk-ringbuffer
[ 1617.608891]     # module: printk_ringbuffer_kunit_test
[ 1617.609041]     1..1
[ 1617.613633]     # test_readerwriter: running for 10000 ms
[ 1617.614796]     # test_readerwriter: start thread 001 (writer)
[ 1617.615562]     # test_readerwriter: start thread 002 (writer)
[ 1617.616399]     # test_readerwriter: start thread 003 (writer)
[ 1617.617911]     # test_readerwriter: start thread 004 (writer)
[ 1617.618763]     # test_readerwriter: start thread 005 (writer)
[ 1617.619067]     # test_readerwriter: start thread 006 (writer)
[ 1617.619368]     # test_readerwriter: start thread 007 (writer)
[ 1617.619650]     # test_readerwriter: start thread 008 (writer)
[ 1617.619921]     # test_readerwriter: start thread 009 (writer)
[ 1617.620211]     # test_readerwriter: start thread 010 (writer)
[ 1617.620462]     # test_readerwriter: starting test
[ 1617.623169]     # test_readerwriter: start thread 011 (writer)
[ 1617.623175]     # test_readerwriter: start reader
[ 1617.624508]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=9 size=1111638594 text=
[ 1617.626400]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=13 size=1111638594 text=
[ 1617.628225]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=17 size=1111638594 text=
[ 1627.681624]     # test_readerwriter: end reader: read=14 seq=17
[ 1627.682370]     # test_readerwriter: completed test
[ 1627.683960]     # test_readerwriter: end thread 011: wrote=10452656
[ 1627.684779]     # test_readerwriter: end thread 010: wrote=13707777
[ 1627.685412]     # test_readerwriter: end thread 009: wrote=10480695
[ 1627.686274]     # test_readerwriter: end thread 008: wrote=10470822
[ 1627.687081]     # test_readerwriter: end thread 007: wrote=13697351
[ 1627.687716]     # test_readerwriter: end thread 006: wrote=10464543
[ 1627.688317]     # test_readerwriter: end thread 005: wrote=10488791
[ 1627.688919]     # test_readerwriter: end thread 004: wrote=10488529
[ 1627.689492]     # test_readerwriter: end thread 003: wrote=13738659
[ 1627.690321]     # test_readerwriter: end thread 002: wrote=13718856
[ 1627.690915]     # test_readerwriter: end thread 001: wrote=13675859
[ 1627.691694]     # test_readerwriter.speed: slow
[ 1627.691753]     not ok 1 test_readerwriter
[ 1627.692512] not ok 1 printk-ringbuffer


[ 1627.693708]     KTAP version 1
[ 1627.694046]     # Subtest: printk-ringbuffer
[ 1627.694385]     # module: printk_ringbuffer_kunit_test
[ 1627.694410]     1..1
[ 1627.695698]     # test_readerwriter: running for 10000 ms
[ 1627.698351]     # test_readerwriter: start thread 001 (writer)
[ 1627.698935]     # test_readerwriter: start thread 002 (writer)
[ 1627.699341]     # test_readerwriter: start thread 003 (writer)
[ 1627.699657]     # test_readerwriter: start thread 004 (writer)
[ 1627.699891]     # test_readerwriter: start thread 005 (writer)
[ 1627.700111]     # test_readerwriter: start thread 006 (writer)
[ 1627.700337]     # test_readerwriter: start thread 007 (writer)
[ 1627.700620]     # test_readerwriter: start thread 008 (writer)
[ 1627.700885]     # test_readerwriter: start thread 009 (writer)
[ 1627.701134]     # test_readerwriter: start thread 010 (writer)
[ 1627.701371]     # test_readerwriter: starting test
[ 1627.701374]     # test_readerwriter: start thread 011 (writer)
[ 1627.705369]     # test_readerwriter: start reader
[ 1627.706115]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=16 size=1111638594 text=
[ 1627.707966]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=20 size=1111638594 text=
[ 1627.710886]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=24 size=16962 text=
[ 1637.917622]     # test_readerwriter: end reader: read=14 seq=27
[ 1637.918372]     # test_readerwriter: completed test
[ 1637.919848]     # test_readerwriter: end thread 011: wrote=10559459
[ 1637.920670]     # test_readerwriter: end thread 010: wrote=13811125
[ 1637.921309]     # test_readerwriter: end thread 009: wrote=10572663
[ 1637.922176]     # test_readerwriter: end thread 008: wrote=10570916
[ 1637.922818]     # test_readerwriter: end thread 007: wrote=13832304
[ 1637.923456]     # test_readerwriter: end thread 006: wrote=10571421
[ 1637.924082]     # test_readerwriter: end thread 005: wrote=10579437
[ 1637.924703]     # test_readerwriter: end thread 004: wrote=13861481
[ 1637.925318]     # test_readerwriter: end thread 003: wrote=10590865
[ 1637.925977]     # test_readerwriter: end thread 002: wrote=13852926
[ 1637.926570]     # test_readerwriter: end thread 001: wrote=13845110
[ 1637.927172]     # test_readerwriter.speed: slow
[ 1637.927202]     not ok 1 test_readerwriter
[ 1637.927611] not ok 1 printk-ringbuffer



[ 1658.412099]     KTAP version 1
[ 1658.412409]     # Subtest: printk-ringbuffer
[ 1658.412784]     # module: printk_ringbuffer_kunit_test
[ 1658.412813]     1..1
[ 1658.416372]     # test_readerwriter: running for 10000 ms
[ 1658.417927]     # test_readerwriter: start thread 001 (writer)
[ 1658.418309]     # test_readerwriter: start thread 002 (writer)
[ 1658.418785]     # test_readerwriter: start thread 003 (writer)
[ 1658.418996]     # test_readerwriter: start thread 004 (writer)
[ 1658.419262]     # test_readerwriter: start thread 005 (writer)
[ 1658.419531]     # test_readerwriter: start thread 006 (writer)
[ 1658.419758]     # test_readerwriter: start thread 007 (writer)
[ 1658.420028]     # test_readerwriter: start thread 008 (writer)
[ 1658.420239]     # test_readerwriter: start thread 009 (writer)
[ 1658.420516]     # test_readerwriter: start thread 010 (writer)
[ 1658.420737]     # test_readerwriter: starting test
[ 1658.420739]     # test_readerwriter: start thread 011 (writer)
[ 1658.420791]     # test_readerwriter: start reader
[ 1658.420879]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=8 size=217 text=DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD\x10\xa2\xff\xff\xff\xff#\x8b^M\xa4\xff\xff\xff\xff\xb3\x8a\xb5A
[ 1658.421015]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=9 size=217 text=DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD\x10\xa2\xff\xff\xff\xff#\x8b^M\xa4\xff\xff\xff\xff\xb3\x8a\xb5A
[ 1658.421152]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=10 size=1145324612 text=
[ 1658.431961]     # test_readerwriter: EXPECTATION FAILED at kernel/printk/printk_ringbuffer_kunit_test.c:80
               BAD RECORD: seq=18 size=1145324612 text=
[ 1668.637609]     # test_readerwriter: end reader: read=13 seq=20
[ 1668.638384]     # test_readerwriter: completed test
[ 1668.639464]     # test_readerwriter: end thread 011: wrote=13482649
[ 1668.640116]     # test_readerwriter: end thread 010: wrote=10354440
[ 1668.640934]     # test_readerwriter: end thread 009: wrote=10388436
[ 1668.642245]     # test_readerwriter: end thread 008: wrote=13621613
[ 1668.642886]     # test_readerwriter: end thread 007: wrote=10072648
[ 1668.643536]     # test_readerwriter: end thread 006: wrote=13582778
[ 1668.644171]     # test_readerwriter: end thread 005: wrote=10397092
[ 1668.644792]     # test_readerwriter: end thread 004: wrote=13499818
[ 1668.645415]     # test_readerwriter: end thread 003: wrote=10362779
[ 1668.646319]     # test_readerwriter: end thread 002: wrote=10348508
[ 1668.646924]     # test_readerwriter: end thread 001: wrote=13616849
[ 1668.647584]     # test_readerwriter.speed: slow
[ 1668.647618]     not ok 1 test_readerwriter
[ 1668.648311] not ok 1 printk-ringbuffer


Best Regards,
Petr