> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuzn...@redhat.com]
> Sent: Monday, July 13, 2015 5:19 AM
> To: de...@linuxdriverproject.org
> Cc: KY Srinivasan; Haiyang Zhang; Dexuan Cui; linux-ker...@vger.kernel.org
> Subject: [PATCH] Drivers: hv: vmbus: prevent new subchannel creation on
> device shutdown
> 
> When a new subchannel offer from host comes during device shutdown
> (e.g.
> when a netvsc/storvsc module is unloadedshortly after it was loaded) a
> crash can happen as vmbus_process_offer() is not anyhow serialized with
> vmbus_remove(). As an example we can try calling subchannel create
> callback when the module is already unloaded.
> The following crash was observed while keeping loading/unloading
> hv_netvsc
> module on 64-CPU guest:
> 
>   hv_netvsc vmbus_14: net device safe to remove
>   BUG: unable to handle kernel paging request at 000000000000a118
>   IP: [<ffffffffa01c1b21>] netvsc_sc_open+0x31/0xb0 [hv_netvsc]
>   PGD 1f3946a067 PUD 1f38a5f067 PMD 0
>   Oops: 0000 [#1] SMP
>   ...
>   Call Trace:
>   [<ffffffffa0055cd7>] vmbus_onoffer+0x477/0x530 [hv_vmbus]
>   [<ffffffff81092e1f>] ? move_linked_works+0x5f/0x80
>   [<ffffffffa0055fd3>] vmbus_onmessage+0x33/0xa0 [hv_vmbus]
>   [<ffffffffa0052f81>] vmbus_onmessage_work+0x21/0x30 [hv_vmbus]
>   [<ffffffff81095cde>] process_one_work+0x18e/0x4e0
>   ...
> 
> The issue cannot be solved by just resetting sc_creation_callback on
> driver removal as while we search for the parent channel with channel_lock
> held we release it after the channel was found and it can disapper beneath
> our feet while we're still in vmbus_process_offer();
> 
> Introduce new sc_create_lock mutex and take it in vmbus_remove() to
> ensure
> no new subchannels are created after we started the removal procedure.
> Check its state with mutex_trylock in vmbus_process_offer().
> 
> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com>

Thanks Vitaly; strangely enough, I too am looking at this exact problem. I was 
planning to
address this problem a little differently: I was planning to hold the context 
that performed
the initial (primary) channel open until all of the "open activity" was 
completed and this would include
opening of the sub-channels for multi-channel devices. 

Regards,

K. Y

> ---
>  drivers/hv/channel.c      |  3 +++
>  drivers/hv/channel_mgmt.c | 20 ++++++++++++++++++--
>  drivers/hv/vmbus_drv.c    |  6 ++++++
>  include/linux/hyperv.h    |  4 ++++
>  4 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
> index 603ce97..faa91fe 100644
> --- a/drivers/hv/channel.c
> +++ b/drivers/hv/channel.c
> @@ -555,6 +555,9 @@ static int vmbus_close_internal(struct vmbus_channel
> *channel)
>       if (channel->rescind)
>               hv_process_channel_removal(channel,
>                                          channel->offermsg.child_relid);
> +     else if (mutex_is_locked(&channel->sc_create_lock))
> +             mutex_unlock(&channel->sc_create_lock);
> +
>       return ret;
>  }
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 4506a66..96f8b96 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -150,6 +150,8 @@ static struct vmbus_channel *alloc_channel(void)
>       INIT_LIST_HEAD(&channel->sc_list);
>       INIT_LIST_HEAD(&channel->percpu_list);
> 
> +     mutex_init(&channel->sc_create_lock);
> +
>       return channel;
>  }
> 
> @@ -158,6 +160,7 @@ static struct vmbus_channel *alloc_channel(void)
>   */
>  static void free_channel(struct vmbus_channel *channel)
>  {
> +     mutex_destroy(&channel->sc_create_lock);
>       kfree(channel);
>  }
> 
> @@ -247,8 +250,18 @@ static void vmbus_process_offer(struct
> vmbus_channel *newchannel)
>                       newchannel->offermsg.offer.if_type) &&
>                       !uuid_le_cmp(channel->offermsg.offer.if_instance,
>                               newchannel->offermsg.offer.if_instance)) {
> -                     fnew = false;
> -                     break;
> +                     if (mutex_trylock(&channel->sc_create_lock)) {
> +                             fnew = false;
> +                             break;
> +                     }
> +                     /*
> +                      * If we fail to acquire mutex here it means we're
> +                      * closing parent channel at this moment. We can't
> +                      * create new subchannel in this case.
> +                      */
> +
>       spin_unlock_irqrestore(&vmbus_connection.channel_lock,
> +                                            flags);
> +                     goto err_free_chan;
>               }
>       }
> 
> @@ -297,6 +310,7 @@ static void vmbus_process_offer(struct
> vmbus_channel *newchannel)
>       if (!fnew) {
>               if (channel->sc_creation_callback != NULL)
>                       channel->sc_creation_callback(newchannel);
> +             mutex_unlock(&channel->sc_create_lock);
>               return;
>       }
> 
> @@ -340,6 +354,8 @@ err_deq_chan:
>       }
> 
>  err_free_chan:
> +     if (!fnew)
> +             mutex_unlock(&channel->sc_create_lock);
>       free_channel(newchannel);
>  }
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index cf20400..7ad7fcc 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -539,6 +539,12 @@ static int vmbus_remove(struct device
> *child_device)
>       struct hv_device *dev = device_to_hv_device(child_device);
>       u32 relid = dev->channel->offermsg.child_relid;
> 
> +     /*
> +      * Device is being removed, prevent creating new subchannels.
> Mutex
> +      * will be released in vmbus_close_internal().
> +      */
> +     mutex_lock(&dev->channel->sc_create_lock);
> +
>       if (child_device->driver) {
>               drv = drv_to_hv_drv(child_device->driver);
>               if (drv->remove)
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index 30d3a1f..f1af05c 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -748,6 +748,10 @@ struct vmbus_channel {
>        */
>       struct vmbus_channel *primary_channel;
>       /*
> +      * Protects sub-channel create path.
> +      */
> +     struct mutex sc_create_lock;
> +     /*
>        * Support per-channel state for use by vmbus drivers.
>        */
>       void *per_channel_state;
> --
> 2.4.3

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