Re: kobject lifetime issues in blk-mq

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Nov 12, 2018 at 05:44:07PM +0800, Ming Lei wrote:
> On Mon, Nov 12, 2018 at 05:20:51PM +0800, Ming Lei wrote:
> > On Fri, Nov 09, 2018 at 12:35:18PM -0800, Guenter Roeck wrote:
> > > Hi,
> > > 
> > > Images with CONFIG_DEBUG_KOBJECT=y, CONFIG_DEBUG_KOBJECT_RELEASE=y
> > > enabled report kobject lifetime issues when booting an image in qemu
> > > using virtio-scsi-pci.
> > > 
> > > Bisect points to two different commits, depending on the kernel
> > > configuration.
> > > 
> > > 1st configuration: defconfig plus:
> > > 
> > > CONFIG_VIRTIO_BLK=y
> > > CONFIG_SCSI_LOWLEVEL=y
> > > CONFIG_SCSI_VIRTIO=y
> > > CONFIG_VIRTIO_PCI=y
> > > CONFIG_VIRTIO_MMIO=y
> > > CONFIG_DEBUG_OBJECTS=y
> > > CONFIG_DEBUG_OBJECTS_FREE=y
> > > CONFIG_DEBUG_OBJECTS_TIMERS=y
> > > CONFIG_DEBUG_KOBJECT=y
> > > CONFIG_DEBUG_KOBJECT_RELEASE=y
> > > 
> > > Bisecting this configuration points to commit b5b6e8c8d3b4 ("scsi:
> > > virtio_scsi: fix IO hang caused by automatic irq vector affinity")
> > > as the culprit. This commit enforces the use of blk-mq for virtio_scsi;
> > > the bisect log is therefore a bit misleading.
> > > 
> > > 2nd configuration: As above, plus:
> > > CONFIG_SCSI_MQ_DEFAULT=y
> > > 
> > > With this configuration, bisect points to commit 7ea5fe31c12d
> > > ("blk-mq: make lifetime consitent between q/ctx and its kobject").
> > > 
> > > The qemu command line used to reproduce the problem is as follows.
> > > The qemu version does not seem to matter (I tested with qemu versions
> > > 2.5, 2.11, and 3.0).
> > > 
> > > qemu-system-x86_64 \
> > > 	-kernel arch/x86/boot/bzImage \
> > > 	-device virtio-scsi-pci,id=scsi \
> > > 	-device scsi-hd,bus=scsi.0,drive=d0 \
> > > 	-drive file=./wheezy.img,format=raw,if=none,id=d0 \
> > > 	-snapshot \
> > > 	-m 2G -smp 4 -enable-kvm \
> > > 	-net user,host=10.0.2.10,hostfwd=tcp::10022-:22 -net nic \
> > > 	-nographic -monitor none \
> > > 	-no-reboot \
> > > 	-append "console=ttyS0 root=/dev/sda earlyprintk=serial panic_on_warn=1 panic=-1"
> > > 
> > > Root file system:
> > > 	https://storage.googleapis.com/syzkaller/wheezy.img
> > > or:
> > > 	https://github.com/groeck/linux-build-test/blob/master/rootfs/x86_64/rootfs.ext2.gz
> > > 
> > > It seems that any root file system can be used to reproduce the problem.
> > > Also, the problem is not limited to virtio-scsi-pci. I also tried
> > > am53c974, lsi53c810, and lsi53c895a (after enabling the respective
> > > drivers), with the same result.
> > > 
> > > Overall, this suggests that the problem is related to blk-mq and was
> > > indeed introduced with commit 7ea5fe31c12d.
> > > 
> > > For reference, I attached an actual crash log as well as a set of bisect
> > > logs below.
> > > 
> > > Unfortunately, my understanding of blk-mq is not good enough to find the
> > > underlying problem and suggest a fix. Please let me know if there is
> > > anything else I can do to help fixing the problem.
> > > 
> > > Note that the problem was originally reported by syzbot running a test
> > > on chromeos-4.14. It may well be that the problem has already been
> > > reported and is being worked on. If so, please apologize the noise.
> > > 
> > > Thanks,
> > > Guenter
> > > 
> > > ---
> > > [    8.700038] ------------[ cut here ]------------
> > > [    8.700376] ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x10
> > > [    8.701032] WARNING: CPU: 0 PID: 0 at lib/debugobjects.c:329 debug_print_object+0x6a/0x80
> > > [    8.701032] Kernel panic - not syncing: panic_on_warn set ...
> > > [    8.701032] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.20.0-rc1 #30
> > > [    8.701032] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> > > [    8.701032] Call Trace:
> > > [    8.701032]  <IRQ>
> > > [    8.701032]  dump_stack+0x46/0x5b
> > > [    8.701032]  panic+0xf3/0x246
> > > [    8.701032]  ? debug_print_object+0x6a/0x80
> > > [    8.701032]  __warn+0xeb/0xf0
> > > [    8.701032]  ? debug_print_object+0x6a/0x80
> > > [    8.701032]  ? debug_print_object+0x6a/0x80
> > > [    8.701032]  report_bug+0xb1/0x120
> > > [    8.701032]  fixup_bug.part.10+0x13/0x30
> > > [    8.701032]  do_error_trap+0x8f/0xb0
> > > [    8.701032]  do_invalid_op+0x31/0x40
> > > [    8.701032]  ? debug_print_object+0x6a/0x80
> > > [    8.701032]  invalid_op+0x14/0x20
> > > [    8.701032] RIP: 0010:debug_print_object+0x6a/0x80
> > > [    8.701032] Code: 8b 43 10 83 c2 01 8b 4b 14 89 15 c1 e2 78 01 4c 8b 45 00 4c
> > > 89 e6 48 c7 c7 f0 ca 41 a6 48 8b 14 c5 a0 f7 24 a6 e8 06 8e cc ff <0f> 0b 5b 83
> > > 05 70 47 1a 01 01 5d 41 5c c3 83 05 65 47 1a 01 01 c3
> > > [    8.701032] RSP: 0018:ffff89b9fda03e48 EFLAGS: 00010082
> > > [    8.701032] RAX: 0000000000000000 RBX: ffff89b9fd49d0c8 RCX: ffffffffa6646e18
> > > [    8.701032] RDX: 0000000000000001 RSI: 0000000000000082 RDI: ffffffffa6cc596c
> > > [    8.701032] RBP: ffffffffa664a5c0 R08: 79616c6564203a74 R09: 5f6b726f775f6465
> > > [    8.701032] R10: ffff89b9fda03f10 R11: 3178302f3078302b R12: ffffffffa6403e22
> > > [    8.701032] R13: 0000000000000001 R14: ffffffffa6d4d458 R15: ffff89b9fc4c8780
> > > [    8.701032]  ? debug_print_object+0x6a/0x80
> > > [    8.701032]  debug_check_no_obj_freed+0x1b8/0x1ea
> > > [    8.701032]  ? rcu_process_callbacks+0x2a7/0x750
> > > [    8.701032]  kmem_cache_free+0x6e/0x1a0
> > > [    8.701032]  ? __blk_release_queue+0x150/0x150
> > > [    8.701032]  rcu_process_callbacks+0x2a7/0x750
> > > [    8.701032]  __do_softirq+0xf2/0x2c7
> > > [    8.701032]  irq_exit+0xb7/0xc0
> > > [    8.701032]  smp_apic_timer_interrupt+0x67/0x130
> > > [    8.701032]  apic_timer_interrupt+0xf/0x20
> > > [    8.701032]  </IRQ>
> > > [    8.701032] RIP: 0010:default_idle+0x12/0x140
> > > [    8.701032] Code: fe ff ff c6 43 08 00 fb eb b0 31 c0 eb b4 e8 65 c8 60 ff 90
> > > 90 90 90 90 41 54 55 53 65 8b 2d 15 c4 3b 5a 66 66 66 66 90 fb f4 <65> 8b 2d 07
> > > c4 3b 5a 66 66 66 66 90 5b 5d 41 5c c3 65 8b 05 f6 c3
> > > [    8.701032] RSP: 0018:ffffffffa6603e98 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
> > > [    8.701032] RAX: ffffffffa5c52d10 RBX: 0000000000000000 RCX: 0000000000000000
> > > [    8.701032] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 000000020605092a
> > > [    8.701032] RBP: 0000000000000000 R08: ffffffffcff7766a R09: ffff89b9fda20b00
> > > [    8.701032] R10: 0000000000000000 R11: 0000000000002400 R12: ffffffffa6611740
> > > [    8.701032] R13: 0000000000000000 R14: 0000000000000000 R15: ffffffffa6611740
> > > [    8.701032]  ? __sched_text_end+0x5/0x5
> > > [    8.701032]  do_idle+0x19c/0x230
> > > [    8.701032]  cpu_startup_entry+0x14/0x20
> > > [    8.701032]  start_kernel+0x491/0x4b1
> > > [    8.701032]  secondary_startup_64+0xa4/0xb0
> > > [    8.701032] Kernel Offset: 0x24200000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> > > 
> > 
> > That is simply caused by enabling DEBUG_KOBJECT_RELEASE itself, which
> > tries to do kboject_release() after delaying a while.
> > 
> > Wrt. this issue, q->mq_kobj is embedded in 'request_queue' whose
> > instance is freed in release handler of q->kobj. So when one instance
> > of 'request_queue' is freed, DEBUG_KOBJECT_RELEASE still expects that
> > the q->mq_kobj is active.
> > 
> > The release handler of q->mq_kobj is NOP actually, so in reality it won't
> > be one issue at all.
> 
> Given we have removed legacy IO path completely, it should be fine to
> remove q->mq_kobj and simply put everything under the kobj of queue,
> especially any attributes under mq aren't marked as stable ABI.

That might be possible going forward, but that does not appear to be
a feasible solution for stable releases. I am not sure though if the
"stable ABI" argument is valid. Almost all sysfs attributes are not
marked stable. If there are applications using it, removing the atttributes
would still break userspace.

Since the release function of mq_kobj is NOP (for reference, I assume
we are talking about blk_mq_sysfs_release), can it just be NULL ?
That it is an empty function seems to be just as good or bad as having
no function at all. An empty function just avoids the "broken" debug
message. That message is still better than rendering DEBUG_KOBJECT_RELEASE
useless.

Copying GregKH for input.

Guenter