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Date: Fri, 26 Apr 2024 12:14:38 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: =?iso-8859-1?Q?Andr=E9?= Almeida <andrealmeid@igalia.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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	"Paul E . McKenney" <paulmck@kernel.org>,
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	Ingo Molnar <mingo@redhat.com>, Darren Hart <dvhart@infradead.org>,
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	Noah Goldstein <goldstein.w.n@gmail.com>,
	Daniel Colascione <dancol@google.com>, longman@redhat.com,
	kernel-dev@igalia.com
Subject: Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation
Message-ID: <20240426101438.GA39183@noisy.programming.kicks-ass.net>
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On Fri, Apr 26, 2024 at 11:43:51AM +0200, Florian Weimer wrote:
> * Andr? Almeida:
> 
> > With FUTEX2_SPIN flag set during a futex_wait(), the futex value is
> > expected to be the PID of the lock owner. Then, the kernel gets the
> > task_struct of the corresponding PID, and checks if it's running. It
> > spins until the futex is awaken, the task is scheduled out or if a
> > timeout happens.  If the lock owner is scheduled out at any time, then
> > the syscall follows the normal path of sleeping as usual.
> 
> PID or TID?

TID, just like PI_LOCK I would presume.

> I think we'd like to have at least one, possibly more, bits for free
> use, so the kernel ID comparison should at least mask off the MSB,
> possibly more.

Yeah, it should be using FUTEX_TID_MASK -- just like PI_LOCK :-)

I suppose the question is if this thing should then also imply
FUTEX_WAITERS or not.