linux-kernel - Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

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Message-ID: <20180328111952.GS4043@hirez.programming.kicks-ass.net>
Date:   Wed, 28 Mar 2018 13:19:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences
 system call (v12)

On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote:
> +#ifdef CONFIG_RSEQ
> +	struct rseq __user *rseq;
> +	u32 rseq_len;
> +	u32 rseq_sig;
> +	/*
> +	 * RmW on rseq_event_mask must be performed atomically
> +	 * with respect to preemption.
> +	 */
> +	unsigned long rseq_event_mask;
> +#endif

> +static inline void rseq_signal_deliver(struct pt_regs *regs)
> +{
> +	set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
> +	rseq_handle_notify_resume(regs);
> +}
> +
> +static inline void rseq_preempt(struct task_struct *t)
> +{
> +	set_bit(RSEQ_EVENT_PREEMPT_BIT, &t->rseq_event_mask);
> +	rseq_set_notify_resume(t);
> +}
> +
> +static inline void rseq_migrate(struct task_struct *t)
> +{
> +	set_bit(RSEQ_EVENT_MIGRATE_BIT, &t->rseq_event_mask);
> +	rseq_set_notify_resume(t);
> +}

Given that comment above, do you really need the full atomic set bit?
Isn't __set_bit() sufficient?