Re: [PATCH] net: hinic: avoid kernel hung in hinic_get_stats64()

[Paolo Abeni]

On Wed, 2022-06-29 at 15:28 +0800, Qiao Ma wrote:
> When using hinic device as a bond slave device, and reading device stats of
> master bond device, the kernel may hung.
> 
> The kernel panic calltrace as follows:
> Kernel panic - not syncing: softlockup: hung tasks
> Call trace:
>   native_queued_spin_lock_slowpath+0x1ec/0x31c
>   dev_get_stats+0x60/0xcc
>   dev_seq_printf_stats+0x40/0x120
>   dev_seq_show+0x1c/0x40
>   seq_read_iter+0x3c8/0x4dc
>   seq_read+0xe0/0x130
>   proc_reg_read+0xa8/0xe0
>   vfs_read+0xb0/0x1d4
>   ksys_read+0x70/0xfc
>   __arm64_sys_read+0x20/0x30
>   el0_svc_common+0x88/0x234
>   do_el0_svc+0x2c/0x90
>   el0_svc+0x1c/0x30
>   el0_sync_handler+0xa8/0xb0
>   el0_sync+0x148/0x180
> 
> And the calltrace of task that actually caused kernel hungs as follows:
>   __switch_to+124
>   __schedule+548
>   schedule+72
>   schedule_timeout+348
>   __down_common+188
>   __down+24
>   down+104
>   hinic_get_stats64+44 [hinic]
>   dev_get_stats+92
>   bond_get_stats+172 [bonding]
>   dev_get_stats+92
>   dev_seq_printf_stats+60
>   dev_seq_show+24
>   seq_read_iter+964
>   seq_read+220
>   proc_reg_read+164
>   vfs_read+172
>   ksys_read+108
>   __arm64_sys_read+28
>   el0_svc_common+132
>   do_el0_svc+40
>   el0_svc+24
>   el0_sync_handler+164
>   el0_sync+324
> 
> When getting device stats from bond, kernel will call bond_get_stats().
> It first holds the spinlock bond->stats_lock, and then call
> hinic_get_stats64() to collect hinic device's stats.
> However, hinic_get_stats64() calls `down(&nic_dev->mgmt_lock)` to
> protect its critical section, which may schedule current task out.
> And if system is under high pressure, the task cannot be woken up
> immediately, which eventually triggers kernel hung panic.
> 
> Fixes: edd384f682cc ("net-next/hinic: Add ethtool and stats")
> Signed-off-by: Qiao Ma <mqaio@xxxxxxxxxxxxxxxxx>

Side note: it looks like that after this patch every section protected
by the mgmt_lock is already under rtnl lock protection, so you could
probably remove the hinic specific lock (in a separate, net-next,
patch).

Please double check the above as I skimmed upon that quickly.

Thanks,

Paolo