In message: [linux-yocto][linux-yocto v5.10][PATCH] eventfd: Enlarge recursion 
limit to allow vhost to work
on 28/06/2021 qiang.zh...@windriver.com wrote:

> From: Zqiang <qiang.zh...@windriver.com>
> 
> 1/1 [
> Author: He Zhe
> Email: zhe...@windriver.com
> Subject: eventfd: Enlarge recursion limit to allow vhost to work
> Date: Fri, 5 Jun 2020 16:32:18 +0800
> 
> commit 85f0a97f3aac6bb2c9549af607843644dd2ef5c7 upstream [linux-yocto]
> 
> Upstream link: 
> https://lore.kernel.org/lkml/20200410114720.24838-1-zhe...@windriver.com/

I was reading the thread on lore, and it isn't clear to me what is
the latest status.

Does this problem show more easily under preempt-rt, but is not a
preempt-rt only fix ? I'm just trying to confirm that you are
targeting this at v5.10/standard/base.

You first submitted this a year ago, and it still isn't applied
upstream. And Juri was seeing the same warning. There was a
recent series posted that had a similar change to your v1, but
you've resent this patch (v2?) as part of that effort .. but still
no answer on the change.

Also note, I just pushed 5.13 as the start of the new release
kernel, since this is in mainline still, I assume I can apply this
to those branches as well.

Bruce

> 
> 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...@windriver.com>
> Signed-off-by: Bruce Ashfield <bruce.ashfi...@gmail.com>
> [CE: backport of 85f0a97f3aac from v5.4 branch of linux-yocto]
> Signed-off-by: Catalin Enache <catalin.ena...@windriver.com>
> ]
> 
> Signed-off-by: Zqiang <qiang.zh...@windriver.com>
> ---
>  fs/eventfd.c            | 2 +-
>  include/linux/eventfd.h | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index df466ef81ddd..7a5cbec9e843 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -71,7 +71,7 @@ __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..db050fb99e0b 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -29,6 +29,8 @@
>  #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;
>  
> -- 
> 2.29.2
> 
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