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From: Thomas Gleixner <tglx@linutronix.de>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Peter Zijlstra
 <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, "Paul E . McKenney" <paulmck@kernel.org>,
 Boqun Feng <boqun.feng@gmail.com>, "H . Peter Anvin" <hpa@zytor.com>, Paul
 Turner <pjt@google.com>, linux-api@vger.kernel.org, Christian Brauner
 <brauner@kernel.org>, Florian Weimer <fw@deneb.enyo.de>,
 David.Laight@ACULAB.COM, carlos@redhat.com, Peter Oskolkov <posk@posk.io>,
 Alexander Mikhalitsyn <alexander@mihalicyn.com>, Chris Kennelly
 <ckennelly@google.com>, Ingo Molnar <mingo@redhat.com>, Darren Hart
 <dvhart@infradead.org>, Davidlohr Bueso <dave@stgolabs.net>, =?utf-8?Q?An?=
 =?utf-8?Q?dr=C3=A9?= Almeida
 <andrealmeid@igalia.com>, libc-alpha@sourceware.org, Steven Rostedt
 <rostedt@goodmis.org>, Jonathan Corbet <corbet@lwn.net>, Noah Goldstein
 <goldstein.w.n@gmail.com>, Daniel Colascione <dancol@google.com>,
 longman@redhat.com, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [RFC PATCH v2 4/4] selftests/rseq: Implement rseq_mutex test
 program
In-Reply-To: <20230529191416.53955-5-mathieu.desnoyers@efficios.com>
References: <20230529191416.53955-1-mathieu.desnoyers@efficios.com>
 <20230529191416.53955-5-mathieu.desnoyers@efficios.com>
Date: Thu, 28 Sep 2023 21:55:20 +0200
Message-ID: <87v8budqdz.ffs@tglx>
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On Mon, May 29 2023 at 15:14, Mathieu Desnoyers wrote:
> +static void rseq_lock_slowpath(struct rseq_mutex *lock)
> +{
> +	int i = 0;
> +
> +	for (;;) {
> +		struct rseq_abi_sched_state *expected = NULL, *self = rseq_get_sched_state(rseq_get_abi());
> +
> +		if (__atomic_compare_exchange_n(&lock->owner, &expected, self, false,
> +						__ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> +			break;
> +		//TODO: use rseq critical section to protect dereference of owner thread's
> +		//rseq_abi_sched_state, combined with rseq fence at thread reclaim.
> +		if ((RSEQ_READ_ONCE(expected->state) & RSEQ_ABI_SCHED_STATE_FLAG_ON_CPU) &&

For robust and PI futexes the futex value is an ABI between kernel and
user space. Sure, this is only meant for demonstration, but it's the
wrong direction.

Thanks,

        tglx