linux-kernel - Re: [RFC 02/10] drm: Update file owner during use

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Message-ID: <77587046-aede-f625-6a35-55bf9c5f1179@linux.intel.com>
Date:   Thu, 8 Jun 2023 15:26:26 +0100
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Emil Velikov <emil.l.velikov@...il.com>
Cc:     Intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Rob Clark <robdclark@...omium.org>,
        Brian Welty <brian.welty@...el.com>, Kenny.Ho@....com,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org,
        Stéphane Marchesin <marcheu@...omium.org>,
        Christian König <christian.koenig@....com>,
        Zefan Li <lizefan.x@...edance.com>,
        Dave Airlie <airlied@...hat.com>, Tejun Heo <tj@...nel.org>,
        cgroups@...r.kernel.org, "T . J . Mercier" <tjmercier@...gle.com>
Subject: Re: [RFC 02/10] drm: Update file owner during use


On 21/04/2023 13:13, Emil Velikov wrote:
> Greetings everyone,
> 
> Above all - hell yeah. Thank you Tvrtko, this has been annoying the
> hell out of me for ages.

Yay!

> On Tue, 14 Mar 2023 at 14:19, Tvrtko Ursulin
> <tvrtko.ursulin@...ux.intel.com> wrote:
>>
>> From: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
>>
>> With the typical model where the display server opends the file descriptor
>> and then hands it over to the client we were showing stale data in
>> debugfs.
> 
> s/opends/opens/

Thanks!

> But as a whole the sentence is fairly misleading. Story time:
> 
> The traditional model, the server was the orchestrator managing the
> primary device node. From the fd, to the master status and
> authentication. But looking at the fd alone, this has varied across
> the years.
> 
> IIRC in the DRI1 days, Xorg (libdrm really) would have a list of open
> fd(s) and reuse those whenever needed, DRI2 the client was responsible
> for open() themselves and with DRI3 the fd was passed to the client.
> 
> Around the inception of DRI3 and systemd-logind, the latter became
> another possible orchestrator. Whereby Xorg and Wayland compositors
> could ask it for the fd. For various reasons (hysterical and genuine
> ones) Xorg has a fallback path going the open(), whereas Wayland
> compositors are moving to solely relying on logind... some never had
> fallback even.
> 
> Over the past few years, more projects have emerged which provide
> functionality similar (be that on API level, Dbus, or otherwise) to
> systemd-logind.
> 
> 
> Apart from that, the commit is spot on. I like the use of rcu and the
> was_master handling is correct. With some message polish this commit
> is:
> Reviewed-by: Emil Velikov <emil.l.velikov@...il.com>

Are you okay if I just paste your very fine explanation verbatim, with 
credits?

> I also had a brief look at 01/10, although I cannot find many
> references for the pid <> tguid mappings. Be that on the kernel side
> or userspace - do you have any links that I can educate myself?

TGID or thread group leader. For single threaded userspace TGID equals 
to PID, while for multi-threaded first thread TGID equals PID/TID, while 
additional threads PID/TID does not equal TGID. Clear, as mud? :) My 
POSIX book is misplaced somewhere having not consulted it years... :)

Regards,

Tvrtko