Re: [PATCH 1/2] eventfd: Make wake counter work for single fd instead of all

[Jens Axboe]

On 4/7/20 3:59 AM, zhe.he@xxxxxxxxxxxxx wrote:
> From: He Zhe <zhe.he@xxxxxxxxxxxxx>
> 
> commit b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> introduces a percpu counter that tracks the percpu recursion depth and
> warn if it greater than one, to avoid potential deadlock and stack
> overflow.
> 
> However sometimes different eventfds may be used in parallel.
> Specifically, when high network load goes through kvm and vhost, working
> as below, it would trigger the following call trace.
> 
> -  100.00%
>    - 66.51%
>         ret_from_fork
>         kthread
>       - vhost_worker
>          - 33.47% handle_tx_kick
>               handle_tx
>               handle_tx_copy
>               vhost_tx_batch.isra.0
>               vhost_add_used_and_signal_n
>               eventfd_signal
>          - 33.05% handle_rx_net
>               handle_rx
>               vhost_add_used_and_signal_n
>               eventfd_signal
>    - 33.49%
>         ioctl
>         entry_SYSCALL_64_after_hwframe
>         do_syscall_64
>         __x64_sys_ioctl
>         ksys_ioctl
>         do_vfs_ioctl
>         kvm_vcpu_ioctl
>         kvm_arch_vcpu_ioctl_run
>         vmx_handle_exit
>         handle_ept_misconfig
>         kvm_io_bus_write
>         __kvm_io_bus_write
>         eventfd_signal
> 
> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> ---- snip ----
> 001: Call Trace:
> 001:  vhost_signal+0x15e/0x1b0 [vhost]
> 001:  vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> 001:  handle_rx+0xb9/0x900 [vhost_net]
> 001:  handle_rx_net+0x15/0x20 [vhost_net]
> 001:  vhost_worker+0xbe/0x120 [vhost]
> 001:  kthread+0x106/0x140
> 001:  ? log_used.part.0+0x20/0x20 [vhost]
> 001:  ? kthread_park+0x90/0x90
> 001:  ret_from_fork+0x35/0x40
> 001: ---[ end trace 0000000000000003 ]---
> 
> This patch moves the percpu counter into eventfd control structure and
> does the clean-ups, so that eventfd can still be protected from deadlock
> while allowing different ones to work in parallel.
> 
> As to potential stack overflow, we might want to figure out a better
> solution in the future to warn when the stack is about to overflow so it
> can be better utilized, rather than break the working flow when just the
> second one comes.

This doesn't work for the infinite recursion case, the state has to be
global, or per thread.

-- 
Jens Axboe