linux-kernel - Re: [RFC PATCH v10 for 4.15 01/14] Restartable sequences system call

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Date:   Tue, 7 Nov 2017 09:24:24 +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 v10 for 4.15 01/14] Restartable sequences system call

On Mon, Nov 06, 2017 at 03:56:31PM -0500, Mathieu Desnoyers wrote:
[...]
> +
> +/*
> + * struct rseq is aligned on 4 * 8 bytes to ensure it is always
> + * contained within a single cache-line.
> + *
> + * A single struct rseq per thread is allowed.
> + */
> +struct rseq {
> +	/*
> +	 * Restartable sequences cpu_id_start field. Updated by the
> +	 * kernel, and read by user-space with single-copy atomicity
> +	 * semantics. Aligned on 32-bit. Always contain a value in the
> +	 * range of possible CPUs, although the value may not be the
> +	 * actual current CPU (e.g. if rseq is not initialized). This
> +	 * CPU number value should always be confirmed against the value
> +	 * of the cpu_id field.
> +	 */
> +	uint32_t cpu_id_start;
> +	/*
> +	 * Restartable sequences cpu_id field. Updated by the kernel,
> +	 * and read by user-space with single-copy atomicity semantics.
> +	 * Aligned on 32-bit. Values -1U and -2U have a special
> +	 * semantic: -1U means "rseq uninitialized", and -2U means "rseq
> +	 * initialization failed".
> +	 */
> +	uint32_t cpu_id;
> +	/*
> +	 * Restartable sequences rseq_cs field.
> +	 *
> +	 * Contains NULL when no critical section is active for the current
> +	 * thread, or holds a pointer to the currently active struct rseq_cs.
> +	 *
> +	 * Updated by user-space at the beginning of assembly instruction
> +	 * sequence block, and by the kernel when it restarts an assembly
> +	 * instruction sequence block, and when the kernel detects that it
> +	 * is preempting or delivering a signal outside of the range
> +	 * targeted by the rseq_cs. Also needs to be cleared by user-space
> +	 * before reclaiming memory that contains the targeted struct
> +	 * rseq_cs.
> +	 *
> +	 * Read and set by the kernel with single-copy atomicity semantics.
> +	 * Aligned on 64-bit.
> +	 */
> +	RSEQ_FIELD_u32_u64(rseq_cs);
> +	/*
> +	 * - RSEQ_DISABLE flag:
> +	 *
> +	 * Fallback fast-track flag for single-stepping.
> +	 * Set by user-space if lack of progress is detected.
> +	 * Cleared by user-space after rseq finish.
> +	 * Read by the kernel.
> +	 * - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
> +	 *     Inhibit instruction sequence block restart and event
> +	 *     counter increment on preemption for this thread.

Nit: "event counter" has been removed entirely ;-)

> +	 * - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
> +	 *     Inhibit instruction sequence block restart and event
> +	 *     counter increment on signal delivery for this thread.
> +	 * - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
> +	 *     Inhibit instruction sequence block restart and event
> +	 *     counter increment on migration for this thread.
> +	 */
> +	uint32_t flags;
> +} __attribute__((aligned(4 * sizeof(uint64_t))));
> +
> +#endif /* _UAPI_LINUX_RSEQ_H */
[...]
> +	} else {
> +		/*
> +		 * If there was no rseq previously registered,
> +		 * we need to ensure the provided rseq is
> +		 * properly aligned and valid.
> +		 */
> +		if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq))
> +				|| rseq_len != sizeof(*rseq))
> +			return -EINVAL;
> +		if (!access_ok(VERIFY_WRITE, rseq, rseq_len))
> +			return -EFAULT;
> +		current->rseq = rseq;
> +		current->rseq_len = rseq_len;
> +		current->rseq_sig = sig;
> +		/*
> +		 * If rseq was previously inactive, and has just
> +		 * been registered, ensure the cpu_id and
> +		 * event_counter fields are updated before

s/event_counter/cpu_start_id/ ?

Regards,
Boqun

> +		 * returning to user-space.
> +		 */
> +		rseq_set_notify_resume(current);
> +	}
> +
> +	return 0;
> +}
[...]

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