linux-kernel - Re: [PATCH 0/2] eventfd: simplify signal helpers

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Message-ID: <20230714-gauner-unsolidarisch-fc51f96c61e8@brauner>
Date:   Fri, 14 Jul 2023 09:05:21 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>
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Subject: Re: [PATCH 0/2] eventfd: simplify signal helpers

On Thu, Jul 13, 2023 at 11:10:54AM -0600, Alex Williamson wrote:
> On Thu, 13 Jul 2023 12:05:36 +0200
> Christian Brauner <brauner@...nel.org> wrote:
> 
> > Hey everyone,
> > 
> > This simplifies the eventfd_signal() and eventfd_signal_mask() helpers
> > by removing the count argument which is effectively unused.
> 
> We have a patch under review which does in fact make use of the
> signaling value:
> 
> https://lore.kernel.org/all/20230630155936.3015595-1-jaz@semihalf.com/

Huh, thanks for the link.

Quoting from
https://patchwork.kernel.org/project/kvm/patch/20230307220553.631069-1-jaz@semihalf.com/#25266856

> Reading an eventfd returns an 8-byte value, we generally only use it
> as a counter, but it's been discussed previously and IIRC, it's possible
> to use that value as a notification value.

So the goal is to pipe a specific value through eventfd? But it is
explicitly a counter. The whole thing is written around a counter and
each write and signal adds to the counter.

The consequences are pretty well described in the cover letter of
v6 https://lore.kernel.org/all/20230630155936.3015595-1-jaz@semihalf.com/

> Since the eventfd counter is used as ACPI notification value
> placeholder, the eventfd signaling needs to be serialized in order to
> not end up with notification values being coalesced. Therefore ACPI
> notification values are buffered and signalized one by one, when the
> previous notification value has been consumed.

But isn't this a good indication that you really don't want an eventfd
but something that's explicitly designed to associate specific data with
a notification? Using eventfd in that manner requires serialization,
buffering, and enforces ordering.

I have no skin in the game aside from having to drop this conversion
which I'm fine to do if there are actually users for this btu really,
that looks a lot like abusing an api that really wasn't designed for
this.