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Subject[PATCH 03/11] Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels
Date
When Hyper-V sends an interrupt to the guest, the guest has to figure
out which channel the interrupt is associated with. Hyper-V sets a bit
in a memory page that is shared with the guest, indicating a particular
"relid" that the interrupt is associated with. The current Linux code
then uses a set of per-CPU linked lists to map a given "relid" to a
pointer to a channel structure.

This design introduces a synchronization problem if the CPU that Hyper-V
will interrupt for a certain channel is changed. If the interrupt comes
on the "old CPU" and the channel was already moved to the per-CPU list
of the "new CPU", then the relid -> channel mapping will fail and the
interrupt is dropped. Similarly, if the interrupt comes on the new CPU
but the channel was not moved to the per-CPU list of the new CPU, then
the mapping will fail and the interrupt is dropped.

Relids are integers ranging from 0 to 2047. The mapping from relids to
channel structures can be done by setting up an array with 2048 entries,
each entry being a pointer to a channel structure (hence total size ~16K
bytes, which is not a problem). The array is global, so there are no
per-CPU linked lists to update. The array can be searched and updated
by loading from/storing to the array at the specified index. With no
per-CPU data structures, the above mentioned synchronization problem is
avoided and the relid2channel() function gets simpler.

Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
NOTE. The inline comment in vmbus_channel_map_relid() follows the argument
discussed in the RFC [1]. An attempt has been made to make this argument
more 'explicit' (and, hopefully, more 'robust' against future changes) by
adding a full memory barrier in vmbus_channel_map_relid(): the barrier takes
the role of the 'implicit' full memory barriers from queue_work() and from
check_ready_for_resume_event() referred to in the RFC.

[1] https://lkml.kernel.org/r/20200403133826.GA25401@andrea

drivers/hv/channel_mgmt.c | 179 ++++++++++++++++++++++++--------------
drivers/hv/connection.c | 38 +++-----
drivers/hv/hv.c | 2 -
drivers/hv/hyperv_vmbus.h | 14 +--
drivers/hv/vmbus_drv.c | 48 ++++++----
include/linux/hyperv.h | 5 --
6 files changed, 160 insertions(+), 126 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index aed8db59a5ff8..8c179f6866ef7 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -319,7 +319,6 @@ static struct vmbus_channel *alloc_channel(void)
init_completion(&channel->rescind_event);

INIT_LIST_HEAD(&channel->sc_list);
- INIT_LIST_HEAD(&channel->percpu_list);

tasklet_init(&channel->callback_event,
vmbus_on_event, (unsigned long)channel);
@@ -340,23 +339,49 @@ static void free_channel(struct vmbus_channel *channel)
kobject_put(&channel->kobj);
}

-static void percpu_channel_enq(void *arg)
+void vmbus_channel_map_relid(struct vmbus_channel *channel)
{
- struct vmbus_channel *channel = arg;
- struct hv_per_cpu_context *hv_cpu
- = this_cpu_ptr(hv_context.cpu_context);
-
- list_add_tail_rcu(&channel->percpu_list, &hv_cpu->chan_list);
+ if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
+ return;
+ /*
+ * The mapping of the channel's relid is visible from the CPUs that
+ * execute vmbus_chan_sched() by the time that vmbus_chan_sched() will
+ * execute:
+ *
+ * (a) In the "normal (i.e., not resuming from hibernation)" path,
+ * the full barrier in smp_store_mb() guarantees that the store
+ * is propagated to all CPUs before the add_channel_work work
+ * is queued. In turn, add_channel_work is queued before the
+ * channel's ring buffer is allocated/initialized and the
+ * OPENCHANNEL message for the channel is sent in vmbus_open().
+ * Hyper-V won't start sending the interrupts for the channel
+ * before the OPENCHANNEL message is acked. The memory barrier
+ * in vmbus_chan_sched() -> sync_test_and_clear_bit() ensures
+ * that vmbus_chan_sched() must find the channel's relid in
+ * recv_int_page before retrieving the channel pointer from the
+ * array of channels.
+ *
+ * (b) In the "resuming from hibernation" path, the smp_store_mb()
+ * guarantees that the store is propagated to all CPUs before
+ * the VMBus connection is marked as ready for the resume event
+ * (cf. check_ready_for_resume_event()). The interrupt handler
+ * of the VMBus driver and vmbus_chan_sched() can not run before
+ * vmbus_bus_resume() has completed execution (cf. resume_noirq).
+ */
+ smp_store_mb(
+ vmbus_connection.channels[channel->offermsg.child_relid],
+ channel);
}

-static void percpu_channel_deq(void *arg)
+void vmbus_channel_unmap_relid(struct vmbus_channel *channel)
{
- struct vmbus_channel *channel = arg;
-
- list_del_rcu(&channel->percpu_list);
+ if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
+ return;
+ WRITE_ONCE(
+ vmbus_connection.channels[channel->offermsg.child_relid],
+ NULL);
}

-
static void vmbus_release_relid(u32 relid)
{
struct vmbus_channel_relid_released msg;
@@ -376,17 +401,25 @@ void hv_process_channel_removal(struct vmbus_channel *channel)
struct vmbus_channel *primary_channel;
unsigned long flags;

- BUG_ON(!mutex_is_locked(&vmbus_connection.channel_mutex));
+ lockdep_assert_held(&vmbus_connection.channel_mutex);
BUG_ON(!channel->rescind);

- if (channel->target_cpu != get_cpu()) {
- put_cpu();
- smp_call_function_single(channel->target_cpu,
- percpu_channel_deq, channel, true);
- } else {
- percpu_channel_deq(channel);
- put_cpu();
- }
+ /*
+ * hv_process_channel_removal() could find INVALID_RELID only for
+ * hv_sock channels. See the inline comments in vmbus_onoffer().
+ */
+ WARN_ON(channel->offermsg.child_relid == INVALID_RELID &&
+ !is_hvsock_channel(channel));
+
+ /*
+ * Upon suspend, an in-use hv_sock channel is removed from the array of
+ * channels and the relid is invalidated. After hibernation, when the
+ * user-space appplication destroys the channel, it's unnecessary and
+ * unsafe to remove the channel from the array of channels. See also
+ * the inline comments before the call of vmbus_release_relid() below.
+ */
+ if (channel->offermsg.child_relid != INVALID_RELID)
+ vmbus_channel_unmap_relid(channel);

if (channel->primary_channel == NULL) {
list_del(&channel->listentry);
@@ -447,16 +480,6 @@ static void vmbus_add_channel_work(struct work_struct *work)

init_vp_index(newchannel, dev_type);

- if (newchannel->target_cpu != get_cpu()) {
- put_cpu();
- smp_call_function_single(newchannel->target_cpu,
- percpu_channel_enq,
- newchannel, true);
- } else {
- percpu_channel_enq(newchannel);
- put_cpu();
- }
-
/*
* This state is used to indicate a successful open
* so that when we do close the channel normally, we
@@ -523,17 +546,10 @@ static void vmbus_add_channel_work(struct work_struct *work)
spin_unlock_irqrestore(&primary_channel->lock, flags);
}

- mutex_unlock(&vmbus_connection.channel_mutex);
+ /* vmbus_process_offer() has mapped the channel. */
+ vmbus_channel_unmap_relid(newchannel);

- if (newchannel->target_cpu != get_cpu()) {
- put_cpu();
- smp_call_function_single(newchannel->target_cpu,
- percpu_channel_deq,
- newchannel, true);
- } else {
- percpu_channel_deq(newchannel);
- put_cpu();
- }
+ mutex_unlock(&vmbus_connection.channel_mutex);

vmbus_release_relid(newchannel->offermsg.child_relid);

@@ -599,6 +615,8 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
spin_unlock_irqrestore(&channel->lock, flags);
}

+ vmbus_channel_map_relid(newchannel);
+
mutex_unlock(&vmbus_connection.channel_mutex);

/*
@@ -940,8 +958,6 @@ static void vmbus_onoffer(struct vmbus_channel_message_header *hdr)
oldchannel = find_primary_channel_by_offer(offer);

if (oldchannel != NULL) {
- atomic_dec(&vmbus_connection.offer_in_progress);
-
/*
* We're resuming from hibernation: all the sub-channel and
* hv_sock channels we had before the hibernation should have
@@ -949,36 +965,65 @@ static void vmbus_onoffer(struct vmbus_channel_message_header *hdr)
* primary channel that we had before the hibernation.
*/

+ /*
+ * { Initially: channel relid = INVALID_RELID,
+ * channels[valid_relid] = NULL }
+ *
+ * CPU1 CPU2
+ *
+ * [vmbus_onoffer()] [vmbus_device_release()]
+ *
+ * LOCK channel_mutex LOCK channel_mutex
+ * STORE channel relid = valid_relid LOAD r1 = channel relid
+ * MAP_RELID channel if (r1 != INVALID_RELID)
+ * UNLOCK channel_mutex UNMAP_RELID channel
+ * UNLOCK channel_mutex
+ *
+ * Forbids: r1 == valid_relid &&
+ * channels[valid_relid] == channel
+ *
+ * Note. r1 can be INVALID_RELID only for an hv_sock channel.
+ * None of the hv_sock channels which were present before the
+ * suspend are re-offered upon the resume. See the WARN_ON()
+ * in hv_process_channel_removal().
+ */
+ mutex_lock(&vmbus_connection.channel_mutex);
+
+ atomic_dec(&vmbus_connection.offer_in_progress);
+
WARN_ON(oldchannel->offermsg.child_relid != INVALID_RELID);
/* Fix up the relid. */
oldchannel->offermsg.child_relid = offer->child_relid;

offer_sz = sizeof(*offer);
- if (memcmp(offer, &oldchannel->offermsg, offer_sz) == 0) {
- check_ready_for_resume_event();
- return;
+ if (memcmp(offer, &oldchannel->offermsg, offer_sz) != 0) {
+ /*
+ * This is not an error, since the host can also change
+ * the other field(s) of the offer, e.g. on WS RS5
+ * (Build 17763), the offer->connection_id of the
+ * Mellanox VF vmbus device can change when the host
+ * reoffers the device upon resume.
+ */
+ pr_debug("vmbus offer changed: relid=%d\n",
+ offer->child_relid);
+
+ print_hex_dump_debug("Old vmbus offer: ",
+ DUMP_PREFIX_OFFSET, 16, 4,
+ &oldchannel->offermsg, offer_sz,
+ false);
+ print_hex_dump_debug("New vmbus offer: ",
+ DUMP_PREFIX_OFFSET, 16, 4,
+ offer, offer_sz, false);
+
+ /* Fix up the old channel. */
+ vmbus_setup_channel_state(oldchannel, offer);
}

- /*
- * This is not an error, since the host can also change the
- * other field(s) of the offer, e.g. on WS RS5 (Build 17763),
- * the offer->connection_id of the Mellanox VF vmbus device
- * can change when the host reoffers the device upon resume.
- */
- pr_debug("vmbus offer changed: relid=%d\n",
- offer->child_relid);
-
- print_hex_dump_debug("Old vmbus offer: ", DUMP_PREFIX_OFFSET,
- 16, 4, &oldchannel->offermsg, offer_sz,
- false);
- print_hex_dump_debug("New vmbus offer: ", DUMP_PREFIX_OFFSET,
- 16, 4, offer, offer_sz, false);
-
- /* Fix up the old channel. */
- vmbus_setup_channel_state(oldchannel, offer);
-
+ /* Add the channel back to the array of channels. */
+ vmbus_channel_map_relid(oldchannel);
check_ready_for_resume_event();

+ mutex_unlock(&vmbus_connection.channel_mutex);
return;
}

@@ -1036,14 +1081,14 @@ static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
*
* CPU1 CPU2
*
- * [vmbus_process_offer()] [vmbus_onoffer_rescind()]
+ * [vmbus_onoffer()] [vmbus_onoffer_rescind()]
*
* LOCK channel_mutex WAIT_ON offer_in_progress == 0
* DECREMENT offer_in_progress LOCK channel_mutex
- * INSERT chn_list SEARCH chn_list
+ * STORE channels[] LOAD channels[]
* UNLOCK channel_mutex UNLOCK channel_mutex
*
- * Forbids: CPU2's SEARCH from *not* seeing CPU1's INSERT
+ * Forbids: CPU2's LOAD from *not* seeing CPU1's STORE
*/

while (atomic_read(&vmbus_connection.offer_in_progress) != 0) {
diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
index f4bd306d2cef9..11170d9a2e1a5 100644
--- a/drivers/hv/connection.c
+++ b/drivers/hv/connection.c
@@ -248,6 +248,14 @@ int vmbus_connect(void)
pr_info("Vmbus version:%d.%d\n",
version >> 16, version & 0xFFFF);

+ vmbus_connection.channels = kcalloc(MAX_CHANNEL_RELIDS,
+ sizeof(struct vmbus_channel *),
+ GFP_KERNEL);
+ if (vmbus_connection.channels == NULL) {
+ ret = -ENOMEM;
+ goto cleanup;
+ }
+
kfree(msginfo);
return 0;

@@ -295,33 +303,9 @@ void vmbus_disconnect(void)
*/
struct vmbus_channel *relid2channel(u32 relid)
{
- struct vmbus_channel *channel;
- struct vmbus_channel *found_channel = NULL;
- struct list_head *cur, *tmp;
- struct vmbus_channel *cur_sc;
-
- BUG_ON(!mutex_is_locked(&vmbus_connection.channel_mutex));
-
- list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
- if (channel->offermsg.child_relid == relid) {
- found_channel = channel;
- break;
- } else if (!list_empty(&channel->sc_list)) {
- /*
- * Deal with sub-channels.
- */
- list_for_each_safe(cur, tmp, &channel->sc_list) {
- cur_sc = list_entry(cur, struct vmbus_channel,
- sc_list);
- if (cur_sc->offermsg.child_relid == relid) {
- found_channel = cur_sc;
- break;
- }
- }
- }
- }
-
- return found_channel;
+ if (WARN_ON(relid >= MAX_CHANNEL_RELIDS))
+ return NULL;
+ return READ_ONCE(vmbus_connection.channels[relid]);
}

/*
diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index e2b3310454640..17bf1f229152b 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -117,8 +117,6 @@ int hv_synic_alloc(void)
pr_err("Unable to allocate post msg page\n");
goto err;
}
-
- INIT_LIST_HEAD(&hv_cpu->chan_list);
}

return 0;
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index ce19a4c583884..41b7267da529a 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -132,12 +132,6 @@ struct hv_per_cpu_context {
* basis.
*/
struct tasklet_struct msg_dpc;
-
- /*
- * To optimize the mapping of relid to channel, maintain
- * per-cpu list of the channels based on their CPU affinity.
- */
- struct list_head chan_list;
};

struct hv_context {
@@ -202,6 +196,8 @@ int hv_ringbuffer_read(struct vmbus_channel *channel,
/* TODO: Need to make this configurable */
#define MAX_NUM_CHANNELS_SUPPORTED 256

+#define MAX_CHANNEL_RELIDS \
+ max(MAX_NUM_CHANNELS_SUPPORTED, HV_EVENT_FLAGS_COUNT)

enum vmbus_connect_state {
DISCONNECTED,
@@ -251,6 +247,9 @@ struct vmbus_connection {
struct list_head chn_list;
struct mutex channel_mutex;

+ /* Array of channels */
+ struct vmbus_channel **channels;
+
/*
* An offer message is handled first on the work_queue, and then
* is further handled on handle_primary_chan_wq or
@@ -337,6 +336,9 @@ int vmbus_add_channel_kobj(struct hv_device *device_obj,

void vmbus_remove_channel_attr_group(struct vmbus_channel *channel);

+void vmbus_channel_map_relid(struct vmbus_channel *channel);
+void vmbus_channel_unmap_relid(struct vmbus_channel *channel);
+
struct vmbus_channel *relid2channel(u32 relid);

void vmbus_free_channels(void);
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 41ec0c95df33f..0e5a98ebe23b1 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1225,33 +1225,39 @@ static void vmbus_chan_sched(struct hv_per_cpu_context *hv_cpu)
if (relid == 0)
continue;

+ /*
+ * Pairs with the kfree_rcu() in vmbus_chan_release().
+ * Guarantees that the channel data structure doesn't
+ * get freed while the channel pointer below is being
+ * dereferenced.
+ */
rcu_read_lock();

/* Find channel based on relid */
- list_for_each_entry_rcu(channel, &hv_cpu->chan_list, percpu_list) {
- if (channel->offermsg.child_relid != relid)
- continue;
+ channel = relid2channel(relid);
+ if (channel == NULL)
+ goto sched_unlock_rcu;

- if (channel->rescind)
- continue;
+ if (channel->rescind)
+ goto sched_unlock_rcu;

- trace_vmbus_chan_sched(channel);
+ trace_vmbus_chan_sched(channel);

- ++channel->interrupts;
+ ++channel->interrupts;

- switch (channel->callback_mode) {
- case HV_CALL_ISR:
- vmbus_channel_isr(channel);
- break;
+ switch (channel->callback_mode) {
+ case HV_CALL_ISR:
+ vmbus_channel_isr(channel);
+ break;

- case HV_CALL_BATCHED:
- hv_begin_read(&channel->inbound);
- /* fallthrough */
- case HV_CALL_DIRECT:
- tasklet_schedule(&channel->callback_event);
- }
+ case HV_CALL_BATCHED:
+ hv_begin_read(&channel->inbound);
+ fallthrough;
+ case HV_CALL_DIRECT:
+ tasklet_schedule(&channel->callback_event);
}

+sched_unlock_rcu:
rcu_read_unlock();
}
}
@@ -2237,9 +2243,12 @@ static int vmbus_bus_suspend(struct device *dev)

list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
/*
- * Invalidate the field. Upon resume, vmbus_onoffer() will fix
- * up the field, and the other fields (if necessary).
+ * Remove the channel from the array of channels and invalidate
+ * the channel's relid. Upon resume, vmbus_onoffer() will fix
+ * up the relid (and other fields, if necessary) and add the
+ * channel back to the array.
*/
+ vmbus_channel_unmap_relid(channel);
channel->offermsg.child_relid = INVALID_RELID;

if (is_hvsock_channel(channel)) {
@@ -2475,6 +2484,7 @@ static void __exit vmbus_exit(void)
hv_debug_rm_all_dir();

vmbus_free_channels();
+ kfree(vmbus_connection.channels);

if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
kmsg_dump_unregister(&hv_kmsg_dumper);
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 692c89ccf5dfd..6c794fd5c903e 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -854,11 +854,6 @@ struct vmbus_channel {
* Support per-channel state for use by vmbus drivers.
*/
void *per_channel_state;
- /*
- * To support per-cpu lookup mapping of relid to channel,
- * link up channels based on their CPU affinity.
- */
- struct list_head percpu_list;

/*
* Defer freeing channel until after all cpu's have
--
2.24.0
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