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Date:   Thu, 13 Jul 2023 07:33:05 -0700
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Subject: Re: [PATCH 2/2] eventfd: simplify eventfd_signal_mask()
From:   Sean Christopherson <seanjc@google.com>
To:     Christian Brauner <brauner@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
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Precedence: bulk

On Thu, Jul 13, 2023, Christian Brauner wrote:
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index dc9e01053235..077be5da72bd 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -43,9 +43,10 @@ struct eventfd_ctx {
>  	int id;
>  };
>  
> -__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
> +bool eventfd_signal_mask(struct eventfd_ctx *ctx, __poll_t mask)
>  {
>  	unsigned long flags;
> +	__u64 n = 1;
>  
>  	/*
>  	 * Deadlock or stack overflow issues can happen if we recurse here
> @@ -68,7 +69,7 @@ __u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
>  	current->in_eventfd = 0;
>  	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
>  
> -	return n;
> +	return n == 1;
>  }

...

> @@ -58,13 +58,12 @@ static inline struct eventfd_ctx *eventfd_ctx_fdget(int fd)
>  	return ERR_PTR(-ENOSYS);
>  }
>  
> -static inline int eventfd_signal(struct eventfd_ctx *ctx)
> +static inline bool eventfd_signal(struct eventfd_ctx *ctx)
>  {
>  	return -ENOSYS;
>  }
>  
> -static inline int eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n,
> -				      unsigned mask)
> +static inline bool eventfd_signal_mask(struct eventfd_ctx *ctx, unsigned mask)
>  {
>  	return -ENOSYS;

This will morph to "true" for what should be an error case.  One option would be
to have eventfd_signal_mask() return 0/-errno instead of the count, but looking
at all the callers, nothing ever actually consumes the result.

KVMGT morphs failure into -EFAULT

	if (vgpu->msi_trigger && eventfd_signal(vgpu->msi_trigger, 1) != 1)
		return -EFAULT;

but the only caller of that user ignores the return value.

	if (vgpu_vreg(vgpu, i915_mmio_reg_offset(GEN8_MASTER_IRQ))
			& ~GEN8_MASTER_IRQ_CONTROL)
		inject_virtual_interrupt(vgpu);

The sample driver in samples/vfio-mdev/mtty.c uses a similar pattern: prints an
error but otherwise ignores the result.

So why not return nothing?  That will simplify eventfd_signal_mask() a wee bit
more, and eliminate that bizarre return value confusion for the ugly stubs, e.g.

void eventfd_signal_mask(struct eventfd_ctx *ctx, unsigned mask)
{
	unsigned long flags;

	/*
	 * Deadlock or stack overflow issues can happen if we recurse here
	 * through waitqueue wakeup handlers. If the caller users potentially
	 * nested waitqueues with custom wakeup handlers, then it should
	 * check eventfd_signal_allowed() before calling this function. If
	 * it returns false, the eventfd_signal() call should be deferred to a
	 * safe context.
	 */
	if (WARN_ON_ONCE(current->in_eventfd))
		return;

	spin_lock_irqsave(&ctx->wqh.lock, flags);
	current->in_eventfd = 1;
	if (ctx->count < ULLONG_MAX)
		ctx->count++;
	if (waitqueue_active(&ctx->wqh))
		wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);
	current->in_eventfd = 0;
	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
}

You could even go further and unify the real and stub versions of eventfd_signal().