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

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Date: Thu, 2 May 2024 12:14:11 +0200
From: Christian Brauner <brauner@...nel.org>
To: Florian Weimer <fweimer@...hat.com>
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

On Thu, May 02, 2024 at 11:51:56AM +0200, Florian Weimer wrote:
> * Christian Brauner:
> 
> > Unless I'm missing something the question here is PID (as in TGID aka
> > thread-group leader id gotten via getpid()) vs TID (thread specific id
> > gotten via gettid()). You want the thread-specific id as you want to
> > interact with the futex state of a specific thread not the thread-group
> > leader.
> >
> > Aside from that TIDs are subject to the same race conditions that PIDs
> > are. They are allocated from the same pool (see alloc_pid()).
> 
> For most mutex types (but not robust mutexes), it is undefined in
> userspace if a thread exits while it has locked a mutex.  Such a usage
> condition would ensure that the race doesn't happen, I believe.

The argument is a bit shaky imho because the race not being able to
happen is predicated on no one being careless enough to exit with a
mutex held. That doesn't do anything against someone doing it on
purpose.

> 
> 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.