Re: [PATCH] vmbus: remove hv_event_tasklet_disable/enable

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Dexuan Cui <decui@microsoft.com> writes:

> With the recent introduction of per-channel tasklet, we need to update
> the way we handle the 3 concurrency issues:
>
> 1. hv_process_channel_removal -> percpu_channel_deq vs.
>    vmbus_chan_sched -> list_for_each_entry(..., percpu_list);
>
> 2. vmbus_process_offer -> percpu_channel_enq/deq vs. vmbus_chan_sched.
>
> 3. vmbus_close_internal vs. the per-channel tasklet vmbus_on_event;
>
> The first 2 issues can be handled by Stephen's recent patch
> "vmbus: use rcu for per-cpu channel list", and the third issue
> can be handled by calling tasklet_disable in vmbus_close_internal here.
>
> We don't need the original hv_event_tasklet_disable/enable since we
> now use per-channel tasklet instead of the previous per-CPU tasklet,
> and actually we must remove them due to the side effect now:
> vmbus_process_offer -> hv_event_tasklet_enable -> tasklet_schedule will
> start the per-channel callback prematurely, cauing NULL dereferencing
> (the channel may haven't been properly configured to run the callback yet).
>
> Fixes: 631e63a9f346 ("vmbus: change to per channel tasklet")
>
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>

Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>

This patch fixes the following crash on boot:

[    1.451648] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[    1.452255] IP: netvsc_channel_cb+0x90/0x7b0 [hv_netvsc]
[    1.452255] PGD 0 
[    1.452255] 
[    1.452255] Oops: 0000 [#1] SMP
[    1.452255] Modules linked in: hv_storvsc hv_netvsc(+) scsi_transport_fc hyperv_fb hyperv_keyboard hid_hyperv hv_vmbus
[    1.452255] CPU: 1 PID: 19 Comm: ksoftirqd/1 Not tainted 4.10.0_test+ #911
[    1.452255] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v1.0 11/26/2012
[    1.452255] task: ffff880007fd2b00 task.stack: ffffc90000e34000
[    1.452255] RIP: 0010:netvsc_channel_cb+0x90/0x7b0 [hv_netvsc]
...
[    1.452255] Call Trace:
[    1.452255]  vmbus_on_event+0x22/0x90 [hv_vmbus]
[    1.452255]  tasklet_action+0x5e/0x110
[    1.452255]  __do_softirq+0x104/0x2af
[    1.452255]  run_ksoftirqd+0x29/0x40
...
[    1.548068] RIP: netvsc_channel_cb+0x90/0x7b0 [hv_netvsc] RSP: ffffc90000e37d88
[    1.548068] CR2: 0000000000000004
[    1.548068] ---[ end trace 601fd9d6588b21e5 ]---
[    1.548068] Kernel panic - not syncing: Fatal exception in interrupt
[    1.548068] Kernel Offset: disabled
[    1.548068] ---[ end Kernel panic - not syncing: Fatal exception in interrupt
[    1.572155] ------------[ cut here ]------------

The crash is not imminent but it happens pretty often on boot, I think
we need to push it to 4.11.

-- 
  Vitaly