linux-kernel - Re: [RFC PATCH 2/3] futex: Add basic infrastructure for local task local hash.

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Message-ID: <20241028104639.GO9767@noisy.programming.kicks-ass.net>
Date: Mon, 28 Oct 2024 11:46:39 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [RFC PATCH 2/3] futex: Add basic infrastructure for local task
 local hash.

On Mon, Oct 28, 2024 at 11:24:08AM +0100, Sebastian Andrzej Siewior wrote:
> On 2024-10-28 11:16:33 [+0100], Peter Zijlstra wrote:
> > On Sun, Oct 27, 2024 at 12:34:51AM +0200, Sebastian Andrzej Siewior wrote:
> > 
> > > Introduce a task local hash map. The hashmap can be allocated via
> > > 	prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_ALLOCATE, 0)
> > 
> > Per process, per task is useless and will make things malfunction.
> > 
> > Things missing in this patch are CLONE_THREAD / CLONE_VM, and create
> > must be absolutely forbidden once mm_users != 1.
> 
> I moved this to struct signal_struct and limited it for now to the
> group leader.

That works I suppose.

'process' is a really dodgy thing in Linux anyway :/

So CLONE_THREAD implies CLONE_SIGHAND, and CLONE_SIGHAND in turn implies
CLONE_VM -- however, you can do CLONE_VM without either SIGHAND or
THREAD, (or SIGHAND|VM without THREAD).

And it all quickly becomes a real mess.

'Sane' userspace doesn't play such games, and insane userspace gets to
keep the pieces I suppose.