Re: io_uring failure on parisc (32-bit userspace and 64-bit kernel)

Jens Axboe
2023-02-12T21:48:48Z

On 2/12/23 1:01?PM, Helge Deller wrote:
> On 2/12/23 20:42, Jens Axboe wrote:
>> On 2/12/23 12:35?PM, Helge Deller wrote:
>>> On 2/12/23 15:03, Helge Deller wrote:
>>>> On 2/12/23 14:35, Jens Axboe wrote:
>>>>> On 2/12/23 6:28?AM, Helge Deller wrote:
>>>>>> On 2/12/23 14:16, Jens Axboe wrote:
>>>>>>> On 2/12/23 2:47?AM, Helge Deller wrote:
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> We see io-uring failures on the parisc architecture with this testcase:
>>>>>>>> https://github.com/axboe/liburing/blob/master/examples/io_uring-test.c
>>>>>>>>
>>>>>>>> parisc is always big-endian 32-bit userspace, with either 32- or 64-bit kernel.
>>>>>>>>
>>>>>>>> On a 64-bit kernel (6.1.11):
>>>>>>>> deller@parisc:~$ ./io_uring-test test.file
>>>>>>>> ret=0, wanted 4096
>>>>>>>> Submitted=4, completed=1, bytes=0
>>>>>>>> -> failure
>>>>>>>>
>>>>>>>> strace shows:
>>>>>>>> io_uring_setup(4, {flags=0, sq_thread_cpu=0, sq_thread_idle=0, sq_entries=4, cq_entries=8, features=IORING_FEAT_SINGLE_MMAP|IORING_FEAT_NODROP|IORING_FEAT_SUBMIT_STABLE|IORING_FEAT_RW_CUR_POS|IORING_FEAT_CUR_PERSONALITY|IORING_FEAT_FAST_POLL|IORING_FEAT_POLL_32BITS|0x1f80, sq_off={head=0, tail=16, ring_mask=64, ring_entries=72, flags=84, dropped=80, array=224}, cq_off={head=32, tail=48, ring_mask=68, ring_entries=76, overflow=92, cqes=96, flags=0x58 /* IORING_CQ_??? */}}) = 3
>>>>>>>> mmap2(NULL, 240, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0) = 0xf7522000
>>>>>>>> mmap2(NULL, 256, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0x10000000) = 0xf6922000
>>>>>>>> openat(AT_FDCWD, "libell0-dbgsym_0.56-2_hppa.deb", O_RDONLY|O_DIRECT) = 4
>>>>>>>> statx(4, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=689308, ...}) = 0
>>>>>>>> getrandom("\x5c\xcf\x38\x2d", 4, GRND_NONBLOCK) = 4
>>>>>>>> brk(NULL) = 0x4ae000
>>>>>>>> brk(0x4cf000) = 0x4cf000
>>>>>>>> io_uring_enter(3, 4, 0, 0, NULL, 8) = 0
>>>>>>>>
>>>>>>>>
>>>>>>>> Running the same testcase on a 32-bit kernel (6.1.11) works:
>>>>>>>> root@debian:~# ./io_uring-test test.file
>>>>>>>> Submitted=4, completed=4, bytes=16384
>>>>>>>> -> ok.
>>>>>>>>
>>>>>>>> strace:
>>>>>>>> io_uring_setup(4, {flags=0, sq_thread_cpu=0, sq_thread_idle=0, sq_entries=4, cq_entries=8, features=IORING_FEAT_SINGLE_MMAP|IORING_FEAT_NODROP|IORING_FEAT_SUBMIT_STABLE|IORING_FEAT_RW_CUR_POS|IORING_FEAT_CUR_PERSONALITY|IORING_FEAT_FAST_POLL|IORING_FEAT_POLL_32BITS|0x1f80, sq_off={head=0, tail=16, ring_mask=64, ring_entries=72, flags=84, dropped=80, array=224}, cq_off={head=32, tail=48, ring_mask=68, ring_entries=76, overflow=92, cqes=96, flags=0x58 /* IORING_CQ_??? */}}) = 3
>>>>>>>> mmap2(NULL, 240, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0) = 0xf6d4c000
>>>>>>>> mmap2(NULL, 256, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, 3, 0x10000000) = 0xf694c000
>>>>>>>> openat(AT_FDCWD, "trace.dat", O_RDONLY|O_DIRECT) = 4
>>>>>>>> statx(4, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=1855488, ...}) = 0
>>>>>>>> getrandom("\xb2\x3f\x0c\x65", 4, GRND_NONBLOCK) = 4
>>>>>>>> brk(NULL) = 0x15000
>>>>>>>> brk(0x36000) = 0x36000
>>>>>>>> io_uring_enter(3, 4, 0, 0, NULL, 8) = 4
>>>>>>>>
>>>>>>>> I'm happy to test any patch if someone has an idea....
>>>>>>>
>>>>>>> No idea what this could be, to be honest. I tried your qemu vm image,
>>>>>>> and it does boot, but it's missing keys to be able to update apt and
>>>>>>> install packages... After fiddling with this for 30 min I gave up, any
>>>>>>> chance you can update the sid image? Given how slow this thing is
>>>>>>> running, it'd take me all day to do a fresh install and I have to admit
>>>>>>> I'm not THAT motivated about parisc to do that :)
>>>>>>
>>>>>> Yes, I will update that image, but qemu currently only supports a
>>>>>> 32-bit PA-RISC CPU which can only run the 32-bit kernel. So even if I
>>>>>> update it, you won't be able to reproduce it, as it only happens with
>>>>>> the 64-bit kernel. I'm sure it's some kind of missing 32-to-64bit
>>>>>> translation in the kernel, which triggers only big-endian machines.
>>>>>
>>>>> I built my own kernel for it, so that should be fine, correct?
>>>>
>>>> No, as qemu won't boot the 64-bit kernel.
>>>>
>>>>> We'll see soon enough, managed to disable enough checks on the
>>>>> debian-10 image to actually make it install packages.
>>>>>
>>>>>> Does powerpc with a 64-bit ppc64 kernel work?
>>>>>> I'd assume it will show the same issue.
>>>>>
>>>>> No idea... Only stuff I use and test on is x86-64/32 and arm64.
>>>>
>>>> Would be interesting if someone could test...
>>>>
>>>>>> I will try to add some printks and compare the output of 32- and
>>>>>> 64-bit kernels. If you have some suggestion where to add such (which?)
>>>>>> debug code, it would help me a lot.
>>>>>
>>>>> I'd just try:
>>>>>
>>>>> echo 1 > /sys/kernel/debug/tracing/events/io_uring
>>>>
>>>> I'll try, but will take some time...
>>>>
>>>
>>> At entry of io_submit_sqes(), io_sqring_entries() returns 0, because
>>> ctx->rings->sq.tail is 0 (wrongly on broken 64-bit, but ok value 4 on 32-bit), and
>>> ctx->cached_sq_head is 0 in both cases.
>>
>> cached_sq_head will get updated as sqes are consumed, but since sq.tail
>> is zero, there's nothing to submit as far as io_uring is concerned.
>>
>> Can you dump addresses/offsets of the sq and cq heads/tails in userspace
>> and in the kernel? They are u32, so same size of 32 and 64-bit.
>
> For both kernels (32- and 64-bit) I get:
> p->sq_off.head = 0 p->sq_off.tail = 16
> p->cq_off.head = 32 p->cq_off.tail = 48

So all that looks as expected. Is it perhaps some mmap thing on 64-bit
kernels? The kernel isn't seeing the updates. You could add the below
debugging, and keep your kernel side stuff. Sounds like they don't quite
agree.

diff --git a/examples/io_uring-test.c b/examples/io_uring-test.c
index 1a685360bff6..f1cfda90c018 100644
--- a/examples/io_uring-test.c
+++ b/examples/io_uring-test.c
@@ -73,7 +73,9 @@ int main(int argc, char *argv[])
break;
} while (1);

+ printf("pre-submit sq head/tail %d/%d, %d/%d\n", *ring.sq.khead, *ring.sq.ktail, ring.sq.sqe_head, ring.sq.sqe_tail);
ret = io_uring_submit(&ring);
+ printf("post-submit sq head/tail %d/%d, %d/%d\n", *ring.sq.khead, *ring.sq.ktail, ring.sq.sqe_head, ring.sq.sqe_tail);
if (ret < 0) {
fprintf(stderr, "io_uring_submit: %s\n", strerror(-ret));
return 1;

--
Jens Axboe