Re: [PATCH] xen: don't reschedule in preemption off sections

[Greg KH <greg@xxxxxxxxx>]

On Thu, Aug 20, 2020 at 08:59:08AM +0200, Juergen Gross wrote:
> For support of long running hypercalls xen_maybe_preempt_hcall() is
> calling cond_resched() in case a hypercall marked as preemptible has
> been interrupted.
> 
> Normally this is no problem, as only hypercalls done via some ioctl()s
> are marked to be preemptible. In rare cases when during such a
> preemptible hypercall an interrupt occurs and any softirq action is
> started from irq_exit(), a further hypercall issued by the softirq
> handler will be regarded to be preemptible, too. This might lead to
> rescheduling in spite of the softirq handler potentially having set
> preempt_disable(), leading to splats like:
> 
> 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: ...
> 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
> 
> Fix that by testing preempt_count() before calling cond_resched().
> 
> In kernel 5.8 this can't happen any more due to the entry code rework
> (more than 100 patches, so not a candidate for backporting).
> 
> The issue was introduced in kernel 4.3, so this patch should go into
> all stable kernels in [4.3 ... 5.7].

Now queued up, thanks!

greg k-h