Re: Linux 5.4.46: BUG: sleeping function called from invalid context at drivers/xen/preempt.c:37

[Jürgen Groß <jgross@xxxxxxxx>]

On 10.07.20 03:38, Sarah Newman wrote:
> BUG: sleeping function called from invalid context at 
> drivers/xen/preempt.c:37
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 20775, name: xl
> INFO: lockdep is turned off.
> CPU: 1 PID: 20775 Comm: xl Tainted: G      D W         
> 5.4.46-1_prgmr_debug.el7.x86_64 #1
> Call Trace:
> <IRQ>
> dump_stack+0x8f/0xd0
> ___might_sleep.cold.76+0xb2/0x103
> xen_maybe_preempt_hcall+0x48/0x70
> xen_do_hypervisor_callback+0x37/0x40
> RIP: e030:xen_hypercall_xen_version+0xa/0x20
> Code: 51 41 53 b8 10 00 00 00 0f 05 41 5b 59 c3 cc cc cc cc cc cc cc cc 
> cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 11 00 00 00 0f 05 <41> 5b 59 
> c3 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> RSP: e02b:ffffc900400dcc30 EFLAGS: 00000246
> RAX: 000000000004000d RBX: 0000000000000200 RCX: ffffffff8100122a
> RDX: ffff88812e788000 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: ffffffff83ee3ad0 R08: 0000000000000001 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000246 R12: ffff8881824aa0b0
> R13: 0000000865496000 R14: 0000000865496000 R15: ffff88815d040000
> ? xen_hypercall_xen_version+0xa/0x20
> ? xen_force_evtchn_callback+0x9/0x10
> ? check_events+0x12/0x20
> ? xen_restore_fl_direct+0x1f/0x20
> ? _raw_spin_unlock_irqrestore+0x53/0x60
> ? debug_dma_sync_single_for_cpu+0x91/0xc0
> ? _raw_spin_unlock_irqrestore+0x53/0x60
> ? xen_swiotlb_sync_single_for_cpu+0x3d/0x140
> ? mlx4_en_process_rx_cq+0x6b6/0x1110 [mlx4_en]
> ? mlx4_en_poll_rx_cq+0x64/0x100 [mlx4_en]
> ? net_rx_action+0x151/0x4a0
> ? __do_softirq+0xed/0x55b
> ? irq_exit+0xea/0x100
> ? xen_evtchn_do_upcall+0x2c/0x40
> ? xen_do_hypervisor_callback+0x29/0x40
> </IRQ>
> ? xen_hypercall_domctl+0xa/0x20
> ? xen_hypercall_domctl+0x8/0x20
> ? privcmd_ioctl+0x221/0x990 [xen_privcmd]
> ? do_vfs_ioctl+0xa5/0x6f0
> ? ksys_ioctl+0x60/0x90
> ? trace_hardirqs_off_thunk+0x1a/0x20
> ? __x64_sys_ioctl+0x16/0x20
> ? do_syscall_64+0x62/0x250
> ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
Sarah, thank you very much for this bug report! I think with your help
I finally found a bug I've been searching for a very long time.

The problem will happen when a preemptible hypercall is interrupted and
a softirq action is triggered issuing further hypercalls and one of
those is preempted, too.

In Linux 5.8 this problem will no longer occur due to the entry code
rework done by Thomas Gleixner. For kernels up to 5.7 I'll write a patch
hopefully repairing this hard to reproduce problem, which IMO more often
will show up with a WARN() splat only in xen_mc_flush().


Juergen