linux-kernel - Re: [syzbot] [kernel?] WARNING in hrtimer

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Message-ID: <87qzwers0e.ffs@tglx>
Date: Wed, 10 Sep 2025 22:07:13 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: syzbot <syzbot+8b3a2e23253b50098164@...kaller.appspotmail.com>,
 anna-maria@...utronix.de, frederic@...nel.org,
 linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Cc: Peter Zijlstra <peterz@...radead.org>, Juri Lelli <jlelli@...hat.com>
Subject: Re: [syzbot] [kernel?] WARNING in hrtimer_forward (4)

On Fri, Aug 29 2025 at 19:00, syzbot wrote:

> HEAD commit:    b6add54ba618 Merge tag 'pinctrl-v6.17-2' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1130eef0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e1e1566c7726877e
> dashboard link: https://syzkaller.appspot.com/bug?extid=8b3a2e23253b50098164
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/102656909b6f/disk-b6add54b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/fa30d1d80a47/vmlinux-b6add54b.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/c25ee8abf30a/bzImage-b6add54b.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8b3a2e23253b50098164@...kaller.appspotmail.com
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 1186 at kernel/time/hrtimer.c:1052 hrtimer_forward+0x1d6/0x2b0 kernel/time/hrtimer.c:1052
> Modules linked in:
> CPU: 1 UID: 0 PID: 1186 Comm: irq/33-virtio1- Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> RIP: 0010:hrtimer_forward+0x1d6/0x2b0 kernel/time/hrtimer.c:1052

It compains that the timer is enqueued when it is attempted to be forwarded

> Code: 4c 89 33 48 8b 04 24 eb 07 e8 86 34 12 00 31 c0 48 83 c4 18 5b 41 5c 41 5d 41 5e 41 5f 5d e9 01 d8 4d 09 cc e8 6b 34 12 00 90 <0f> 0b 90 eb df 48 89 e8 4c 09 f8 48 c1 e8 20 74 0a 48 89 e8 31 d2
> RSP: 0018:ffffc90000a78bd0 EFLAGS: 00010006
> RAX: ffffffff81ac27e5 RBX: ffff8880b883b508 RCX: ffff888026c19dc0
> RDX: 0000000000000100 RSI: 0000000000010000 RDI: 0000000000010100
> RBP: 000000000009d057 R08: 0000000000010000 R09: 0000000000010100
> R10: dffffc0000000000 R11: ffffffff8167a890 R12: ffff8880b883b520
> R13: 0000000000184487 R14: 1ffff110171076a4 R15: 0000000000000001
> FS:  0000000000000000(0000) GS:ffff8881269c2000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f95323cbf98 CR3: 0000000064088000 CR4: 00000000003526f0
> Call Trace:
>  <IRQ>
>  hrtimer_forward_now include/linux/hrtimer.h:366 [inline]
>  dl_server_timer kernel/sched/deadline.c:1193 [inline]

which is strange as this is with the timer callback itself, so it
shouldn't be enqueued, unless there is a possiblilty to have:

   CPU0                       CPU1
                                
   timer_expires()
      callback()              ????
        dl_task_timer()       rq_lock()
          rq_lock()             hrtimer_start()
                              rq_unlock()
           hrtimer_forward()

No idea whether that's possible, but that's the only sensible
explanation.

>  dl_task_timer+0xa42/0x12d0 kernel/sched/deadline.c:1234
>  __run_hrtimer kernel/time/hrtimer.c:1761 [inline]
>  __hrtimer_run_queues+0x503/0xd40 kernel/time/hrtimer.c:1825
>  hrtimer_interrupt+0x45d/0xa90 kernel/time/hrtimer.c:1887
>  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1039 [inline]
>  __sysvec_apic_timer_interrupt+0x10b/0x410 arch/x86/kernel/apic/apic.c:1056
>  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
>  sysvec_apic_timer_interrupt+0xa1/0xc0 arch/x86/kernel/apic/apic.c:1050
>  </IRQ>
>  <TASK>
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702