Re: qemu-x86_64 booting with 8.0.0 stil see int3: when running LTP tracing testing.

[Richard W.M. Jones]

On Wed, Jun 21, 2023 at 06:06:55PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 21, 2023 at 05:31:15PM +0200, Arnd Bergmann wrote:
> 
> > I don't know much about x86 exception handling, but my guess is
> > that this is where the stack overflows, so this backtrace
> > is not all that useful. Looking at the full log from your link,
> > I see that recursion through asm_exc_int3:
> > 
> > <4>[   49.886694]  ? exc_int3+0x62/0x80
> > <4>[   49.886714]  ? asm_exc_int3+0x3e/0x50
> > <4>[   49.886759]  ? preempt_count_sub+0x5/0x80
> > <4>[   49.886783]  ? preempt_count_sub+0x5/0x80
> > <4>[   49.886805]  ? irq_work_queue+0x40/0x80
> > <4>[   49.886826]  ? defer_console_output+0x49/0x80
> > <4>[   49.886840]  ? vprintk+0x42/0x60
> > <4>[   49.886857]  ? _printk+0x5d/0x80
> > <4>[   49.886891]  ? die+0x9c/0xe0
> > <4>[   49.886922]  ? exc_int3+0x62/0x80
> > <4>[   49.886942]  ? asm_exc_int3+0x3e/0x50
> 
> So this is exc_int3(), but it is past poke_int3_handler() which is the
> counter-part of text_poke_bp_batch() -- die() seems to suggest we're
> even past do_int3().
> 
> The fact that it continued past poke_int3_handler() suggests that it is
> a 'stray' int3, not covered by an active text modification site.
> 
> > <4>[   49.886966]  ? __pfx_tick_sched_timer+0x10/0x10
> > <4>[   49.886980]  ? __pfx_read_tsc+0x10/0x10
> > <4>[   49.887010]  ? preempt_count_sub+0x5/0x80
> > <4>[   49.887035]  ? preempt_count_sub+0x5/0x80
> > <4>[   49.887057]  ? __hrtimer_run_queues+0xee/0x330
> > <4>[   49.887070]  ? _raw_spin_unlock_irqrestore+0x28/0x50
> > <4>[   49.887085]  ? __hrtimer_run_queues+0xee/0x330
> > <4>[   49.887136]  ? hrtimer_interrupt+0xf6/0x390
> > <4>[   49.887167]  ? __sysvec_apic_timer_interrupt+0x64/0x1a0
> > <4>[   49.887189]  ? sysvec_apic_timer_interrupt+0x7a/0x90
> > <4>[   49.887206]  </IRQ>
> > <4>[   49.887219]  <TASK>
> > <4>[   49.887231]  ? asm_sysvec_apic_timer_interrupt+0x1f/0x30
> > <4>[   49.887252]  ? sched_rt_period_timer+0x4/0x390
> > <4>[   49.887272]  ? __pfx_do_sync_core+0x10/0x10
> > <4>[   49.887305]  ? insn_get_displacement+0x9/0x160
> > <4>[   49.887329]  ? insn_get_displacement+0x9/0x160
> > <4>[   49.887344]  ? insn_get_immediate+0xd2/0x270
> > <4>[   49.887365]  ? insn_decode+0x113/0x150
> > <4>[   49.887384]  ? text_poke_loc_init+0xea/0x220
> > <4>[   49.887429]  ? sched_rt_period_timer+0x4/0x390
> > <4>[   49.887451]  ? text_poke_queue+0x89/0xa0
> 
> This is the text_poke_queue() stage, it is still collecting sites to
> poke but hasn't actually started yet. The actual poking happens in
> text_poke_bp_batch(), which isn't in the call-chain afaict.
> 
> > <4>[   49.887477]  ? ftrace_replace_code+0x149/0x1f0
> > <4>[   49.887508]  ? ftrace_modify_all_code+0x71/0x140
> > <4>[   49.887533]  ? arch_ftrace_update_code+0xd/0x20
> > <4>[   49.887550]  ? ftrace_shutdown+0xf5/0x220
> > <4>[   49.887577]  ? unregister_ftrace_function+0x2e/0x150
> > <4>[   49.887608]  ? stack_trace_sysctl+0x82/0xb0
> > <4>[   49.887636]  ? proc_sys_call_handler+0x18b/0x280
> > <4>[   49.887675]  ? proc_sys_write+0x17/0x20
> > <4>[   49.887692]  ? vfs_write+0x324/0x3f0
> > <4>[   49.887742]  ? ksys_write+0x75/0xe0
> > <4>[   49.887771]  ? __x64_sys_write+0x1f/0x30
> > <4>[   49.887787]  ? do_syscall_64+0x48/0xa0
> > <4>[   49.887802]  ? sysvec_apic_timer_interrupt+0x4d/0x90
> > <4>[   49.887822]  ? entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> > <4>[   49.887878]  </TASK>

Did anything happen with this bug later?  I see nothing on any public
list, nor any public bug tracker.

We have been having the same sort of problem
(https://bugzilla.redhat.com/show_bug.cgi?id=2216496).  It's another
of those bugs that requires hundreds or thousands of boot iterations
before you can see it.  There is a test in comment 27 but it requires
guestfish and some hacking to work.  I'll try to come up with a
cleaner test later.

We see stack traces like:

[    3.081939] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns
[    3.082266] clocksource: Switched to clocksource acpi_pm
[    3.090201] NET: Registered PF_INET protocol family
[    3.093098] int3: 0000 [#1] PREEMPT SMP NOPTI
[    3.093098] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 6.4.0-10173-ga901a3568fd2 #8
[    3.093098] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
[    3.093098] RIP: 0010:__mod_timer+0x1c3/0x370
[    3.093098] Code: 00 00 41 bd ff ff ff ff 31 d2 4c 89 f6 4c 89 ff e8 f2 ef ff ff 41 89 c4 85 c0 75 09 83 e3 01 0f 85 54 ff ff ff 41 8b 4f 20 66 <90> f7 c1 00 00 10 00 0f 84 23 01 00 00 48 c7 c3 40 cc 01 00 65 48
[    3.093098] RSP: 0018:ffffaf1600013e00 EFLAGS: 00000046
[    3.093098] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000280003
[    3.093098] RDX: 0000000000000000 RSI: ffff9aa90fd9dec0 RDI: ffffffff8441e4b8
[    3.093098] RBP: 00000000fffc200d R08: ffffffff8441e4a0 R09: ffffffff8441e4b8
[    3.093098] R10: 0000000000000001 R11: 000000000002e990 R12: 0000000000000000
[    3.093098] R13: 00000000ffffffff R14: ffff9aa90fd9dec0 R15: ffffffff8441e4b8
[    3.093098] FS:  0000000000000000(0000) GS:ffff9aa90fd80000(0000) knlGS:0000000000000000
[    3.093098] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    3.093098] CR2: 0000000000000000 CR3: 000000004e02e000 CR4: 0000000000750ee0
[    3.093098] PKRU: 55555554
[    3.093098] Call Trace:
[    3.093098]  <TASK>
[    3.093098]  ? die+0x31/0x80
[    3.093098]  ? exc_int3+0x10e/0x120
[    3.093098]  ? asm_exc_int3+0x39/0x40
[    3.093098]  ? __mod_timer+0x1c3/0x370
[    3.093098]  ? __mod_timer+0x1c3/0x370
[    3.093098]  queue_delayed_work_on+0x23/0x30
[    3.093098]  neigh_table_init+0x1bb/0x2e0
[    3.093098]  arp_init+0x12/0x50
[    3.093098]  inet_init+0x15b/0x2f0
[    3.093098]  ? __pfx_inet_init+0x10/0x10
[    3.093098]  do_one_initcall+0x58/0x230
[    3.093098]  kernel_init_freeable+0x199/0x2d0
[    3.093098]  ? __pfx_kernel_init+0x10/0x10
[    3.093098]  kernel_init+0x15/0x1b0
[    3.093098]  ret_from_fork+0x2c/0x50
[    3.093098]  </TASK>
[    3.093098] Modules linked in:
[    3.093098] ---[ end trace 0000000000000000 ]---
[    3.093098] RIP: 0010:__mod_timer+0x1c3/0x370
[    3.093098] Code: 00 00 41 bd ff ff ff ff 31 d2 4c 89 f6 4c 89 ff e8 f2 ef ff ff 41 89 c4 85 c0 75 09 83 e3 01 0f 85 54 ff ff ff 41 8b 4f 20 66 <90> f7 c1 00 00 10 00 0f 84 23 01 00 00 48 c7 c3 40 cc 01 00 65 48
[    3.093098] RSP: 0018:ffffaf1600013e00 EFLAGS: 00000046
[    3.093098] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000280003
[    3.093098] RDX: 0000000000000000 RSI: ffff9aa90fd9dec0 RDI: ffffffff8441e4b8
[    3.093098] RBP: 00000000fffc200d R08: ffffffff8441e4a0 R09: ffffffff8441e4b8
[    3.093098] R10: 0000000000000001 R11: 000000000002e990 R12: 0000000000000000
[    3.093098] R13: 00000000ffffffff R14: ffff9aa90fd9dec0 R15: ffffffff8441e4b8
[    3.093098] FS:  0000000000000000(0000) GS:ffff9aa90fd80000(0000) knlGS:0000000000000000
[    3.093098] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    3.093098] CR2: 0000000000000000 CR3: 000000004e02e000 CR4: 0000000000750ee0
[    3.093098] PKRU: 55555554
[    3.093098] Kernel panic - not syncing: Fatal exception in interrupt

There are many variations, but the common pattern seems to be
<something in the clock or timer code> -> int3 exception

It only happens under qemu TCG (software emulation).

It goes away if we recompile qemu without MTTCG support.

It only happens with -smp enabled, we are using qemu -smp 4

We are using qemu-system-x86_64 full system emulation on x86_64 host
(ie. forcing KVM off).

It happens with the latest upstream kernel and qemu, compiled from
source.

I'd love to have a better way to debug this or collect more
diagnostics if you have any suggestions.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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