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Message-ID: <875xw44fxk.fsf@oldenburg.str.redhat.com>
Date: Fri, 26 Apr 2024 11:43:51 +0200
From: Florian Weimer <fweimer@...hat.com>
To: André Almeida <andrealmeid@...lia.com>
Cc: 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, "Christian
Brauner" <brauner@...nel.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
* André 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?
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.
I haven't really thought about the proposed locking protocol, sorry.
Thanks,
Florian
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