netdev - Re: [RFC v4 01/11] eventfd: Increase the recursion depth of eventfd

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Message-ID: <22e96bd6-0113-ef01-376e-0776d7bdbcd8@redhat.com>
Date:   Tue, 2 Mar 2021 14:44:02 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xie Yongji <xieyongji@...edance.com>, mst@...hat.com,
        stefanha@...hat.com, sgarzare@...hat.com, parav@...dia.com,
        bob.liu@...cle.com, hch@...radead.org, rdunlap@...radead.org,
        willy@...radead.org, viro@...iv.linux.org.uk, axboe@...nel.dk,
        bcrl@...ck.org, corbet@....net
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC v4 01/11] eventfd: Increase the recursion depth of
 eventfd_signal()


On 2021/2/23 7:50 下午, Xie Yongji wrote:
> Increase the recursion depth of eventfd_signal() to 1. This
> is the maximum recursion depth we have found so far.
>
> Signed-off-by: Xie Yongji <xieyongji@...edance.com>


Acked-by: Jason Wang <jasowang@...hat.com>

It might be useful to explain how/when we can reach for this condition.

Thanks


> ---
>   fs/eventfd.c            | 2 +-
>   include/linux/eventfd.h | 5 ++++-
>   2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index e265b6dd4f34..cc7cd1dbedd3 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_DEPTH))
>   		return 0;
>   
>   	spin_lock_irqsave(&ctx->wqh.lock, flags);
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index fa0a524baed0..886d99cd38ef 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)
>   
> +/* Maximum recursion depth */
> +#define EFD_WAKE_DEPTH 1
> +
>   struct eventfd_ctx;
>   struct file;
>   
> @@ -47,7 +50,7 @@ DECLARE_PER_CPU(int, eventfd_wake_count);
>   
>   static inline bool eventfd_signal_count(void)
>   {
> -	return this_cpu_read(eventfd_wake_count);
> +	return this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH;
>   }
>   
>   #else /* CONFIG_EVENTFD */