linux-kernel - Re: [patch V3 05/12] rseq: Add prctl() to enable time slice extensions

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Message-ID: <878qgqsrwz.ffs@tglx>
Date: Fri, 31 Oct 2025 22:05:48 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney"
 <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet
 <corbet@....net>, Prakash Sangappa <prakash.sangappa@...cle.com>, Madadi
 Vineeth Reddy <vineethr@...ux.ibm.com>, K Prateek Nayak
 <kprateek.nayak@....com>, Steven Rostedt <rostedt@...dmis.org>, Sebastian
 Andrzej Siewior <bigeasy@...utronix.de>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org
Subject: Re: [patch V3 05/12] rseq: Add prctl() to enable time slice extensions

On Fri, Oct 31 2025 at 15:43, Mathieu Desnoyers wrote:
> On 2025-10-29 09:22, Thomas Gleixner wrote:
>> +	case PR_RSEQ_SLICE_EXTENSION_SET: {
>> +		u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
>> +		bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE);
>> +
>> +		if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE)
>> +			return -EINVAL;
>> +		if (!rseq_slice_extension_enabled())
>> +			return -ENOTSUPP;
>> +		if (!current->rseq.usrptr)
>> +			return -ENXIO;
>> +
>
> So what happens if we have an (unlikely) scenario of:
>
> - thread startup
> - thread registration to rseq
> - prctl PR_RSEQ_SLICE_EXTENSION_SET
> - rseq unregistration
> - rseq registration
> --> What's the status of slice extension here ?

On unregister it's cleared and you have to re-register it when you
register a new rseq. It's part of the rseq state so obviously it's all
set back to zero.