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
