linux-kernel - Re: [RFC PATCH 0/1] Add FUTEX

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Message-ID: <871q6kmra1.fsf@oldenburg.str.redhat.com>
Date: Thu, 02 May 2024 12:39:34 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: André Almeida <andrealmeid@...lia.com>,  Mathieu
 Desnoyers
 <mathieu.desnoyers@...icios.com>,  Peter Zijlstra <peterz@...radead.org>,
  Thomas Gleixner <tglx@...utronix.de>,  linux-kernel@...r.kernel.org,
  "Paul E . McKenney" <paulmck@...nel.org>,  Boqun Feng
 <boqun.feng@...il.com>,  "H . Peter Anvin" <hpa@...or.com>,  Paul Turner
 <pjt@...gle.com>,  linux-api@...r.kernel.org,  David.Laight@...lab.com,
  carlos@...hat.com,  Peter Oskolkov <posk@...k.io>,  Alexander Mikhalitsyn
 <alexander@...alicyn.com>,  Chris Kennelly <ckennelly@...gle.com>,  Ingo
 Molnar <mingo@...hat.com>,  Darren Hart <dvhart@...radead.org>,  Davidlohr
 Bueso <dave@...olabs.net>,  libc-alpha@...rceware.org,  Steven Rostedt
 <rostedt@...dmis.org>,  Jonathan Corbet <corbet@....net>,  Noah Goldstein
 <goldstein.w.n@...il.com>,  Daniel Colascione <dancol@...gle.com>,
  longman@...hat.com,  kernel-dev@...lia.com
Subject: Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation

* Christian Brauner:

>> From a glibc perspective, we typically cannot use long-term file
>> descriptors (that are kept open across function calls) because some
>> applications do not expect them, or even close them behind our back.
>
> Yeah, good point. Note, I suggested it as an extension not as a
> replacement for the TID. I still think it would be a useful extension in
> general.

Applications will need a way to determine when it is safe to close the
pidfd, though.  If we automate this in glibc (in the same way we handle
thread stack deallocation for example), I think we are essentially back
to square one, except that pidfd collisions are much more likely than
TID collisions, especially on systems that have adjusted kernel.pid_max.
(File descriptor allocation is designed to maximize collisions, after
all.)

Thanks,
Florian