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Message-Id: <1423041265.10558.6@smtp.corp.redhat.com>
Date: Wed, 04 Feb 2015 09:22:25 +0008
From: Jason Wang <jasowang@...hat.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>,
devel@...uxdriverproject.org,
Haiyang Zhang <haiyangz@...rosoft.com>,
linux-kernel@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>
Subject: Re: [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage
workflow for vmbus channels
On Wed, Feb 4, 2015 at 1:00 AM, Vitaly Kuznetsov <vkuznets@...hat.com>
wrote:
> free_channel() function frees the channel unconditionally so we need
> to make
> sure nobody has any link to it. This is not trivial and there are
> several
> examples of races we have:
>
> 1) In vmbus_onoffer_rescind() we check for channel existence with
> relid2channel() and then use it. This can go wrong if we're in the
> middle
> of channel removal (free_channel() was already called).
>
> 2) In process_chn_event() we check for channel existence with
> pcpu_relid2channel() and then use it. This can also go wrong.
>
> 3) vmbus_free_channels() just frees all channels, in case we're in
> the middle
> of vmbus_process_rescind_offer() crash is possible.
>
> The issue can be solved by holding vmbus_connection.channel_lock
> everywhere,
> however, it looks like a way to deadlocks and performance
> degradation. Get/put
> workflow fits here the best.
>
> Implement vmbus_get_channel()/vmbus_put_channel() pair instead of
> free_channel().
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> drivers/hv/channel_mgmt.c | 45
> ++++++++++++++++++++++++++++++++++++++-------
> drivers/hv/connection.c | 7 +++++--
> drivers/hv/hyperv_vmbus.h | 4 ++++
> include/linux/hyperv.h | 13 +++++++++++++
> 4 files changed, 60 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 36bacc7..eb9ce94 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -147,6 +147,8 @@ static struct vmbus_channel *alloc_channel(void)
> return NULL;
>
> channel->id = atomic_inc_return(&chan_num);
> + atomic_set(&channel->count, 1);
> +
> spin_lock_init(&channel->inbound_lock);
> spin_lock_init(&channel->lock);
>
> @@ -178,19 +180,47 @@ static void release_channel(struct work_struct
> *work)
> }
>
> /*
> - * free_channel - Release the resources used by the vmbus channel
> object
> + * vmbus_put_channel - Decrease the channel usage counter and
> release the
> + * resources when this counter reaches zero.
> */
> -static void free_channel(struct vmbus_channel *channel)
> +void vmbus_put_channel(struct vmbus_channel *channel)
> {
> + unsigned long flags;
>
> /*
> * We have to release the channel's workqueue/thread in the vmbus's
> * workqueue/thread context
> * ie we can't destroy ourselves.
> */
> - INIT_WORK(&channel->work, release_channel);
> - queue_work(vmbus_connection.work_queue, &channel->work);
> + spin_lock_irqsave(&channel->lock, flags);
> + if (atomic_dec_and_test(&channel->count)) {
> + channel->dying = true;
> + INIT_WORK(&channel->work, release_channel);
> + spin_unlock_irqrestore(&channel->lock, flags);
> + queue_work(vmbus_connection.work_queue, &channel->work);
> + } else
> + spin_unlock_irqrestore(&channel->lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(vmbus_put_channel);
> +
> +/* vmbus_get_channel - Get additional reference to the channel */
> +struct vmbus_channel *vmbus_get_channel(struct vmbus_channel
> *channel)
> +{
> + unsigned long flags;
> + struct vmbus_channel *ret = NULL;
> +
> + if (!channel)
> + return NULL;
> +
> + spin_lock_irqsave(&channel->lock, flags);
> + if (!channel->dying) {
> + atomic_inc(&channel->count);
> + ret = channel;
> + }
> + spin_unlock_irqrestore(&channel->lock, flags);
Looks like we can use atomic_inc_return_safe() here to avoid extra
dying. And then there's also no need for the spinlock.
if (atomic_inc_return_safe(&channel->count) > 0)
return channel;
else
return NULL;
>
> + return ret;
> }
> +EXPORT_SYMBOL_GPL(vmbus_get_channel);
>
> static void percpu_channel_enq(void *arg)
> {
> @@ -253,7 +283,7 @@ static void vmbus_process_rescind_offer(struct
> work_struct *work)
> list_del(&channel->sc_list);
> spin_unlock_irqrestore(&primary_channel->lock, flags);
> }
> - free_channel(channel);
> + vmbus_put_channel(channel);
> }
>
> void vmbus_free_channels(void)
> @@ -262,7 +292,7 @@ void vmbus_free_channels(void)
>
> list_for_each_entry(channel, &vmbus_connection.chn_list, listentry)
> {
> vmbus_device_unregister(channel->device_obj);
> - free_channel(channel);
> + vmbus_put_channel(channel);
> }
> }
>
> @@ -391,7 +421,7 @@ done_init_rescind:
> spin_unlock_irqrestore(&newchannel->lock, flags);
> return;
> err_free_chan:
> - free_channel(newchannel);
> + vmbus_put_channel(newchannel);
> }
>
> enum {
> @@ -549,6 +579,7 @@ static void vmbus_onoffer_rescind(struct
> vmbus_channel_message_header *hdr)
> queue_work(channel->controlwq, &channel->work);
>
> spin_unlock_irqrestore(&channel->lock, flags);
> + vmbus_put_channel(channel);
> }
>
> /*
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index c4acd1c..d1ce134 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -247,7 +247,8 @@ void vmbus_disconnect(void)
> * Map the given relid to the corresponding channel based on the
> * per-cpu list of channels that have been affinitized to this CPU.
> * This will be used in the channel callback path as we can do this
> - * mapping in a lock-free fashion.
> + * mapping in a lock-free fashion. Takes additional reference to the
> + * channel, all users are supposed to do vmbus_put_channel().
> */
> static struct vmbus_channel *pcpu_relid2channel(u32 relid)
> {
> @@ -263,7 +264,7 @@ static struct vmbus_channel
> *pcpu_relid2channel(u32 relid)
> }
> }
>
> - return found_channel;
> + return vmbus_get_channel(found_channel);
> }
>
> /*
> @@ -297,6 +298,7 @@ struct vmbus_channel *relid2channel(u32 relid)
> }
> }
> }
> + found_channel = vmbus_get_channel(found_channel);
> spin_unlock_irqrestore(&vmbus_connection.channel_lock, flags);
>
> return found_channel;
> @@ -360,6 +362,7 @@ static void process_chn_event(u32 relid)
> pr_err("no channel callback for relid - %u\n", relid);
> }
>
> + vmbus_put_channel(channel);
> }
>
> /*
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index b055e53..40d70f0 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -687,6 +687,10 @@ void vmbus_device_unregister(struct hv_device
> *device_obj);
> /* VmbusChildDeviceDestroy( */
> /* struct hv_device *); */
>
> +/*
> + * Get the channel by its relid. Takes additional reference to the
> channel so
> + * all users are supposed to do vmbus_put_channel() when they're
> done.
> + */
> struct vmbus_channel *relid2channel(u32 relid);
>
> void vmbus_free_channels(void);
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index e73cfeb..c576d2d 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -649,6 +649,9 @@ struct vmbus_channel {
> /* Unique channel id */
> int id;
>
> + /* Active reference count */
> + atomic_t count;
> +
> struct list_head listentry;
>
> struct hv_device *device_obj;
> @@ -666,6 +669,7 @@ struct vmbus_channel {
> u8 monitor_bit;
>
> bool rescind; /* got rescind msg */
> + bool dying; /* channel is dying */
>
> u32 ringbuffer_gpadlhandle;
>
> @@ -907,6 +911,15 @@ extern int vmbus_recvpacket_raw(struct
> vmbus_channel *channel,
>
> extern void vmbus_ontimer(unsigned long data);
>
> +/*
> + * Decrease reference count for the channel. Frees the channel when
> its usage
> + * count reaches zero.
> + */
> +extern void vmbus_put_channel(struct vmbus_channel *channel);
> +
> +/* Get additional reference to the channel */
> +extern struct vmbus_channel *vmbus_get_channel(struct vmbus_channel
> *channel);
> +
> /* Base driver object */
> struct hv_driver {
> const char *name;
> --
> 1.9.3
>
--
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