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Date:   Wed, 30 Nov 2022 15:34:00 +0000
To:     =?utf-8?Q?Andr=C3=A9_Almeida?= <andrealmeid@igalia.com>
From:   Simon Ser <contact@emersion.fr>
Cc:     Pekka Paalanen <ppaalanen@gmail.com>,
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        "Pierre-Loup A . Griffais" <pgriffais@valvesoftware.com>
Subject: Re: [PATCH v3 0/2] drm: Add GPU reset sysfs
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On Wednesday, November 30th, 2022 at 16:23, Andr=C3=A9 Almeida <andrealmeid=
@igalia.com> wrote:

> On 11/28/22 06:30, Simon Ser wrote:
>=20
> > The PID is racy, the user-space daemon could end up killing an
> > unrelated process=E2=80=A6 Is there any way we could use a pidfd instea=
d?
>=20
> Is the PID race condition something that really happens or rather
> something theoretical?

A PID race can happen in practice if many PIDs get spawned. On Linux
PIDs wrap around pretty quickly.

Note, even a sandboxed program inside its own PID namespace can trigger
the wrap-around.

> Anyway, I can't see how pidfd and uevent would work together. Since
> uevent it's kind of a broadcast and pidfd is an anon file, it wouldn't
> be possible to say to userspace which is the fd to be used giving that
> file descriptors are per process resources.

Yeah, I can see how this can be difficult to integrate with uevent.

> On the other hand, this interface could be converted to be an ioctl that
> userspace would block waiting for a reset notification, then the kernel
> could create a pidfd and give to the blocked process the right fd. We
> would probably need a queue to make sure no event is lost.

A blocking IOCTL wouldn't be very nice, you can't integrate that into
an event loop for instance=E2=80=A6