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

[Eric Dumazet]

On Sun, Nov 19, 2023 at 10:25 AM Haifeng Xu <haifeng.xu@xxxxxxxxxx> 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. 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>
> ---

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>