Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work

[He Zhe]

Can this be considered for this moment?
This is actually v2 of
"[PATCH 1/2] eventfd: Make wake counter work for single fd instead of all".

Thanks,
Zhe

On 4/10/20 7:47 PM, 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 zero, to avoid potential deadlock and stack
> overflow.
>
> However sometimes different eventfds may be used in parallel. Specifically,
> when heavy 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 enlarges the limit to 1 which is the maximum recursion depth we
> have found so far.
>
> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
> ---
>  fs/eventfd.c            | 3 ++-
>  include/linux/eventfd.h | 3 +++
>  2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 78e41c7c3d05..8b9bd6fb08cd 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -70,7 +70,8 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
>  	 * it returns true, the eventfd_signal() call should be deferred to a
>  	 * safe context.
>  	 */
> -	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> +	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) >
> +	    EFD_WAKE_COUNT_MAX))
>  		return 0;
>  
>  	spin_lock_irqsave(&ctx->wqh.lock, flags);
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index dc4fd8a6644d..e7684d768e3f 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -29,6 +29,9 @@
>  #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
>  #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
>  
> +/* This is the maximum recursion depth we find so far */
> +#define EFD_WAKE_COUNT_MAX 1
> +
>  struct eventfd_ctx;
>  struct file;
>