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Date:   Fri, 3 Jul 2020 10:12:09 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     zhe.he@windriver.com
Cc:     viro@zeniv.linux.org.uk, axboe@kernel.dk,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] eventfd: Enlarge recursion limit to allow vhost to work
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References: <20200410114720.24838-1-zhe.he@windriver.com>
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Hi,

On 10/04/20 19:47, zhe.he@windriver.com wrote:
> From: He Zhe <zhe.he@windriver.com>
> 
> 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@windriver.com>
> ---

Not sure if this approch can fly, but I also encountered the same
warning (which further caused hangs during VM install) and this change
addresses that.

I'd be interested in understanding what is the status of this problem/fix.

On a side note, by looking at the code, I noticed that (apart from
samples) all callers don't actually check eventfd_signal() return value
and I'm wondering why is that the case and if is it safe to do so.

Thanks,

Juri