linux-kernel - Re: [patch 07/12] rseq: Implement syscall entry work for time slice extensions

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Message-ID: <87plbyu4r1.ffs@tglx>
Date: Wed, 10 Sep 2025 09:49:06 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: K Prateek Nayak <kprateek.nayak@....com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, "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>, Steven Rostedt <rostedt@...dmis.org>, Sebastian
 Andrzej Siewior <bigeasy@...utronix.de>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org
Subject: Re: [patch 07/12] rseq: Implement syscall entry work for time slice
 extensions

On Wed, Sep 10 2025 at 10:52, K. Prateek Nayak wrote:
> On 9/9/2025 4:30 AM, Thomas Gleixner wrote:
>> +static inline void rseq_slice_set_need_resched(struct task_struct *curr)
>> +{
>> +	/*
>> +	 * The interrupt guard is required to prevent inconsistent state in
>> +	 * this case:
>> +	 *
>> +	 * set_tsk_need_resched()
>> +	 * --> Interrupt
>> +	 *       wakeup()
>> +	 *        set_tsk_need_resched()
>> +	 *	  set_preempt_need_resched()
>> +	 *     schedule_on_return()
>> +	 *        clear_tsk_need_resched()
>> +	 *	  clear_preempt_need_resched()
>> +	 * set_preempt_need_resched()		<- Inconsistent state
>> +	 *
>> +	 * This is safe vs. a remote set of TIF_NEED_RESCHED because that
>> +	 * only sets the already set bit and does not create inconsistent
>> +	 * state.
>> +	 */
>> +	scoped_guard(irq)
>> +		set_need_resched_current();
>
> nit. any specific reason for using a scoped_guard() instead of just a
> guard() here (and in rseq_cancel_slice_extension_timer()) other than to
> prominently highlight what is being guarded?

Yes, the intention was to highlight it and scoped_guard() really
does. From a code generation perspective it's the same outcome.

Thanks,

        tglx