Re: [PATCH] kcov, usb: Fix invalid context sleep in softirq path on PREEMPT_RT

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jul 25, 2025 at 08:14:01PM +0000, Yunseong Kim wrote:
> When fuzzing USB with syzkaller on a PREEMPT_RT enabled kernel, following
> bug is triggered in the ksoftirqd context.
> 
> | BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> | in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 30, name: ksoftirqd/1
> | preempt_count: 0, expected: 0
> | RCU nest depth: 2, expected: 2
> | CPU: 1 UID: 0 PID: 30 Comm: ksoftirqd/1 Tainted: G        W           6.16.0-rc1-rt1 #11 PREEMPT_RT
> | Tainted: [W]=WARN
> | Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8 05/13/2025
> | Call trace:
> |  show_stack+0x2c/0x3c (C)
> |  __dump_stack+0x30/0x40
> |  dump_stack_lvl+0x148/0x1d8
> |  dump_stack+0x1c/0x3c
> |  __might_resched+0x2e4/0x52c
> |  rt_spin_lock+0xa8/0x1bc
> |  kcov_remote_start+0xb0/0x490
> |  __usb_hcd_giveback_urb+0x2d0/0x5e8
> |  usb_giveback_urb_bh+0x234/0x3c4
> |  process_scheduled_works+0x678/0xd18
> |  bh_worker+0x2f0/0x59c
> |  workqueue_softirq_action+0x104/0x14c
> |  tasklet_action+0x18/0x8c
> |  handle_softirqs+0x208/0x63c
> |  run_ksoftirqd+0x64/0x264
> |  smpboot_thread_fn+0x4ac/0x908
> |  kthread+0x5e8/0x734
> |  ret_from_fork+0x10/0x20

Why is this only a USB thing?  What is unique about it to trigger this
issue?

thanks,

greg k-h