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@xxxxxxxxxx>
---
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);
+ 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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