[PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for vmbus channels
Vitaly Kuznetsov
vkuznets at redhat.com
Wed Feb 4 09:32:26 UTC 2015
Dexuan Cui <decui at microsoft.com> writes:
>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets at redhat.com]
>> Sent: Wednesday, February 4, 2015 1:01 AM
>> To: KY Srinivasan; devel at linuxdriverproject.org
>> Cc: Haiyang Zhang; linux-kernel at vger.kernel.org; Dexuan Cui; Jason Wang
>> Subject: [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for
>> vmbus channels
>>
>> 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 at redhat.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);
>> + 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;
>> --
>
> Hi Vitaly,
> Thanks for the patchset!
> I once tried to do the same work, but just didn't find enough time. :-)
>
> Please see my below questions:
Thanks for your detailed review! I should have added 'RFC' to my series :-)
>
> Since we always get channel->lock when accessing channel->count, I don't
> think the counter has to be of atomic_t?
Agreed
>
> I suggest we add vmbus_get/put_channel() in vmbus_open/close().
> In this way, IMO patch 3/4 and 4/4 would be unnecessary?
>
I was thinking about that but I wasn't sure it is going to be enough for
sub-channels. I'll look again and report.
> In vmbus_exit(), I suspect we should swap the order of the 2 lines:
> vmbus_free_channels();
> bus_unregister(&hv_bus);
> I suppose in bus_unregister(), the .remove callbacks of all the drivers are
> invoked, and vmbus_close() is invoked for every channel.
Makes sense!
>
> BTW, I just noticed: alloc_channel() -> alloc_workqueue(,,max_active==1,):
> due to max_active==1 here, a channel's process_offer() and
> process_rescind_offer() is already serialized.
> I'm not sure if this fact can be used to simplify the logic?
Workers are serialized but e.g. vmbus_onoffer_rescind() is not
serialized with them, e.g. there is no protection against channel
dessapearance after we found it with relid2channel() (e.g. we're on
failure path in vmbus_process_offer()).
Anyway, we have another examples of (theoretically) possible issues and
get/put workflow should simplify the syncronization.
> Is the previous commit "Drivers: hv: vmbus: serialize Offer and Rescind offer"
> necessary?
It is as not checking work function is bad anyway as (in theory) we can
run vmbus_process_offer() twice if vmbus_onoffer_rescind() runs before
vmbus_process_offer() was started.
>
> BTW2, I think there is an unsafe race in the 2 paths (this is an existing issue,
> not related to this patch of yours):
>
> vmbus_on_event() -> process_chn_event() -> pcpu_relid2channel ->
> list_for_each_entry(channel, pcpu_head, percpu_list)
>
> and
>
> vmbus_process_rescind_offer() -> percpu_channel_deq() ->
> list_del(&channel->percpu_list)
>
> because the former is running in tasklet and hence can preempt the latter (running
> in process context). percpu_channel_enq() should have the same
> issue(?)
As I said in my cover letter I don't believe my series solves all issues :-)
>
> We should add local_bh_disable()/enable() accordingly?
> Can you please help to fix this?
Yes, sure, I'll take a look.
>
> vmbus_exit() -> vmbus_free_channels() -> vmbus_put_channel() may have
> a race with vmbus_process_rescind_offer()?
> After vmbus_process_rescind_offer() -> vmbus_device_unregister(), the count
> can already be the initial 1, later vmbus_free_channels() -> vmbus_put_channel() can
> kfree the channel before vmbus_process_rescind_offer() completely
> finishes?
I was going to rewrite vmbus_free_channels() not only because of
that. E.g. vmbus_get_outgoing_channel() presumes all lists (in both
channel.sc_list and vmbus_connection.chn_list) are valid and we don't
take care of that. Not that I believe vmbus_get_outgoing_channel()
may happen when vmbus_free_channels() is active (and we call
vmbus_free_channels() only if we unload the module)
Thanks for your thorough review, it gave me further inspiration :-)
--
Vitaly
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