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

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Date:   Wed, 28 Mar 2018 14:47:54 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.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:
[...]
> Changes since v11:
> 
> - Replace task struct rseq_preempt, rseq_signal, and rseq_migrate
>   bool by u32 rseq_event_mask.
[...]
> @@ -979,6 +980,17 @@ struct task_struct {
>  	unsigned long			numa_pages_migrated;
>  #endif /* CONFIG_NUMA_BALANCING */
>  
> +#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;

s/unsigned long/u32

> +#endif
> +
>  	struct tlbflush_unmap_batch	tlb_ubc;
>  
>  	struct rcu_head			rcu;
> @@ -1688,4 +1700,110 @@ extern long sched_getaffinity(pid_t pid, struct cpumask *mask);
>  #define TASK_SIZE_OF(tsk)	TASK_SIZE
>  #endif
>  

[...]

> +
> +static int rseq_ip_fixup(struct pt_regs *regs)
> +{
> +	unsigned long ip = instruction_pointer(regs), start_ip = 0,
> +		post_commit_offset = 0, abort_ip = 0;
> +	struct task_struct *t = current;
> +	uint32_t cs_flags = 0;
> +	bool in_rseq_cs = false;

This seems unnecessary? Because..

> +	int ret;
> +
> +	ret = rseq_get_rseq_cs(t, &start_ip, &post_commit_offset, &abort_ip,
> +			&cs_flags);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * Handle potentially not being within a critical section.
> +	 * Unsigned comparison will be true when
> +	 * ip >= start_ip, and when ip < start_ip + post_commit_offset.
> +	 */
> +	if (ip - start_ip < post_commit_offset)
> +		in_rseq_cs = true;
> +
> +	/*
> +	 * If not nested over a rseq critical section, restart is
> +	 * useless. Clear the rseq_cs pointer and return.
> +	 */
> +	if (!in_rseq_cs)
> +		return clear_rseq_cs(t);

we can write

	if (ip - start_ip >= post_commit_offset)
		return clear_rseq_cs(t);

Regards,
Boqun

> +	ret = rseq_need_restart(t, cs_flags);
> +	if (ret <= 0)
> +		return ret;
> +	ret = clear_rseq_cs(t);
> +	if (ret)
> +		return ret;
> +	trace_rseq_ip_fixup(ip, start_ip, post_commit_offset, abort_ip);
> +	instruction_pointer_set(regs, (unsigned long)abort_ip);
> +	return 0;
> +}
> +
[...]

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