Re: [PATCH v2] bonding: use a read-write lock in bonding_show_bonds()

[Haifeng Xu <haifeng.xu@shopee.com>]

On 2023/11/17 18:59, Eric Dumazet wrote:
> On Fri, Nov 17, 2023 at 11:43 AM Haifeng Xu <haifeng.xu@shopee.com> wrote:
>>
>> 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.
>>
>> 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.
>>
>> 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@shopee.com>
>> ---
>> v2:
>> - move the call stack after the description
>> - fix typos in the changelog
>> ---
>>  drivers/net/bonding/bond_main.c  | 4 ++++
>>  drivers/net/bonding/bond_sysfs.c | 6 ++++--
>>  include/net/bonding.h            | 3 +++
>>  3 files changed, 11 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..e107c1d7a6bf 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 +42,7 @@ static ssize_t bonding_show_bonds(const struct class *cls,
>>         int res = 0;
>>         struct bonding *bond;
>>
>> -       rtnl_lock();
>> +       read_lock(&bonding_dev_lock);
>>
>>         list_for_each_entry(bond, &bn->dev_list, bond_list) {
>>                 if (res > (PAGE_SIZE - IFNAMSIZ)) {
>> @@ -55,7 +57,7 @@ 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);
>>         return res;
>>  }
> 
> This unfortunately would race with dev_change_name()

dev_change_name（）is either used in  dev_ifsioc(case: SIOCSIFNAME) or used in do_setlink(), so 
could these net devices which need to change name be related to bond？ I am not quite sure.

> 
> You probably need to read_lock(&devnet_rename_sem); before copying dev->name,
> or use netdev_get_name(net, temp_buffer, bond->dev->ifindex)