[PATCH v3 2/2] bonding: use a read-write lock in bonding_show_bonds()

[Haifeng Xu]

Problem description:

Call stack:
......
PID: 210933  TASK: ffff92424e5ec080  CPU: 13  COMMAND: "kworker/u96:2"
[ffffa7a8e96bbac0] __schedule at ffffffffb0719898
[ffffa7a8e96bbb48] schedule at ffffffffb0719e9e
[ffffa7a8e96bbb68] rwsem_down_write_slowpath at ffffffffafb3167a
[ffffa7a8e96bbc00] down_write at ffffffffb071bfc1
[ffffa7a8e96bbc18] kernfs_remove_by_name_ns at ffffffffafe3593e
[ffffa7a8e96bbc48] sysfs_unmerge_group at ffffffffafe38922
[ffffa7a8e96bbc68] dpm_sysfs_remove at ffffffffb021c96a
[ffffa7a8e96bbc80] device_del at ffffffffb0209af8
[ffffa7a8e96bbcd0] netdev_unregister_kobject at ffffffffb04a6b0e
[ffffa7a8e96bbcf8] unregister_netdevice_many at ffffffffb046d3d9
[ffffa7a8e96bbd60] default_device_exit_batch at ffffffffb046d8d1
[ffffa7a8e96bbdd0] ops_exit_list at ffffffffb045e21d
[ffffa7a8e96bbe00] cleanup_net at ffffffffb045ea46
[ffffa7a8e96bbe60] process_one_work at ffffffffafad94bb
[ffffa7a8e96bbeb0] worker_thread at ffffffffafad96ad
[ffffa7a8e96bbf10] kthread at ffffffffafae132a
[ffffa7a8e96bbf50] ret_from_fork at ffffffffafa04b92

290858 PID: 278176  TASK: ffff925deb39a040  CPU: 32  COMMAND: "node-exporter"
[ffffa7a8d14dbb80] __schedule at ffffffffb0719898
[ffffa7a8d14dbc08] schedule at ffffffffb0719e9e
[ffffa7a8d14dbc28] schedule_preempt_disabled at ffffffffb071a24e
[ffffa7a8d14dbc38] __mutex_lock at ffffffffb071af28
[ffffa7a8d14dbcb8] __mutex_lock_slowpath at ffffffffb071b1a3
[ffffa7a8d14dbcc8] mutex_lock at ffffffffb071b1e2
[ffffa7a8d14dbce0] rtnl_lock at ffffffffb047f4b5
[ffffa7a8d14dbcf0] bonding_show_bonds at ffffffffc079b1a1 [bonding]
[ffffa7a8d14dbd20] class_attr_show at ffffffffb02117ce
[ffffa7a8d14dbd30] sysfs_kf_seq_show at ffffffffafe37ba1
[ffffa7a8d14dbd50] kernfs_seq_show at ffffffffafe35c07
[ffffa7a8d14dbd60] seq_read_iter at ffffffffafd9fce0
[ffffa7a8d14dbdc0] kernfs_fop_read_iter at ffffffffafe36a10
[ffffa7a8d14dbe00] new_sync_read at ffffffffafd6de23
[ffffa7a8d14dbe90] vfs_read at ffffffffafd6e64e
[ffffa7a8d14dbed0] ksys_read at ffffffffafd70977
[ffffa7a8d14dbf10] __x64_sys_read at ffffffffafd70a0a
[ffffa7a8d14dbf20] do_syscall_64 at ffffffffb070bf1c
[ffffa7a8d14dbf50] entry_SYSCALL_64_after_hwframe at ffffffffb080007c
......

Thread 210933 holds the rtnl_mutex and tries to acquire the kernfs_rwsem,
but there are many readers which hold the kernfs_rwsem, so it has to sleep
for a long time to wait the readers release the lock. Thread 278176 and any
other threads which call bonding_show_bonds() also need to wait because
they try to acquire the rtnl_mutex.

bonding_show_bonds() uses rtnl_mutex to protect the bond_list traversal.
However, the addition and deletion of bond_list are only performed in
bond_init()/bond_uninit(), so we can introduce a separate read-write lock
to synchronize bond list mutation. In addition, bonding_show_bonds() could
race with dev_change_name(), so we need devnet_rename_sem to protect the
access to dev->name.

What are the benefits of this change?

1) All threads which call bonding_show_bonds() only wait when the
registration or unregistration of bond device happens or the name
of net device changes.

2) There are many other users of rtnl_mutex, so bonding_show_bonds()
won't compete with them.

In a word, this change reduces the lock contention of rtnl_mutex.

Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
---
v2:
- move the call stack after the description
- fix typos in the changelog
v3:
- add devnet_rename_sem in bonding_show_bonds()
- update the changelog
---
 drivers/net/bonding/bond_main.c  | 4 ++++
 drivers/net/bonding/bond_sysfs.c | 8 ++++++--
 include/net/bonding.h            | 3 +++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 8e6cc0e133b7..db8f1efaab78 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5957,7 +5957,9 @@ static void bond_uninit(struct net_device *bond_dev)
 
 	bond_set_slave_arr(bond, NULL, NULL);
 
+	write_lock(&bonding_dev_lock);
 	list_del(&bond->bond_list);
+	write_unlock(&bonding_dev_lock);
 
 	bond_debug_unregister(bond);
 }
@@ -6370,7 +6372,9 @@ static int bond_init(struct net_device *bond_dev)
 	spin_lock_init(&bond->stats_lock);
 	netdev_lockdep_set_classes(bond_dev);
 
+	write_lock(&bonding_dev_lock);
 	list_add_tail(&bond->bond_list, &bn->dev_list);
+	write_unlock(&bonding_dev_lock);
 
 	bond_prepare_sysfs_group(bond);
 
diff --git a/drivers/net/bonding/bond_sysfs.c b/drivers/net/bonding/bond_sysfs.c
index 2805135a7205..5de71af7c36f 100644
--- a/drivers/net/bonding/bond_sysfs.c
+++ b/drivers/net/bonding/bond_sysfs.c
@@ -28,6 +28,8 @@
 
 #define to_bond(cd)	((struct bonding *)(netdev_priv(to_net_dev(cd))))
 
+DEFINE_RWLOCK(bonding_dev_lock);
+
 /* "show" function for the bond_masters attribute.
  * The class parameter is ignored.
  */
@@ -40,7 +42,8 @@ static ssize_t bonding_show_bonds(const struct class *cls,
 	int res = 0;
 	struct bonding *bond;
 
-	rtnl_lock();
+	down_read(&devnet_rename_sem);
+	read_lock(&bonding_dev_lock);
 
 	list_for_each_entry(bond, &bn->dev_list, bond_list) {
 		if (res > (PAGE_SIZE - IFNAMSIZ)) {
@@ -55,7 +58,8 @@ static ssize_t bonding_show_bonds(const struct class *cls,
 	if (res)
 		buf[res-1] = '\n'; /* eat the leftover space */
 
-	rtnl_unlock();
+	read_unlock(&bonding_dev_lock);
+	up_read(&devnet_rename_sem);
 	return res;
 }
 
diff --git a/include/net/bonding.h b/include/net/bonding.h
index 6c16d778b615..ede4116457e2 100644
--- a/include/net/bonding.h
+++ b/include/net/bonding.h
@@ -783,6 +783,9 @@ extern const u8 lacpdu_mcast_addr[];
 /* exported from net/core/dev.c */
 extern struct rw_semaphore devnet_rename_sem;
 
+/* exported from bond_sysfs.c */
+extern rwlock_t bonding_dev_lock;
+
 static inline netdev_tx_t bond_tx_drop(struct net_device *dev, struct sk_buff *skb)
 {
 	dev_core_stats_tx_dropped_inc(dev);
-- 
2.25.1