linux-kernel - Re: [PATCH V2 3/4] vmbus: add per-channel sysfs info

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Message-ID: <20181018171953.079f6fb6.olaf@aepfle.de>
Date:   Thu, 18 Oct 2018 17:19:53 +0200
From:   Olaf Hering <olaf@...fle.de>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     kys@...rosoft.com, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org
Subject: Re: [PATCH V2 3/4] vmbus: add per-channel sysfs info

Am Sun, 17 Sep 2017 20:54:18 -0700
schrieb kys@...hange.microsoft.com:

> This extends existing vmbus related sysfs structure to provide per-channel
> state information. This is useful when diagnosing issues with multiple
> queues in networking and storage.

> +++ b/drivers/hv/vmbus_drv.c
> +static ssize_t write_avail_show(const struct vmbus_channel *channel, char *buf)
> +{
> +	const struct hv_ring_buffer_info *rbi = &channel->outbound;
> +
> +	return sprintf(buf, "%u\n", hv_get_bytes_to_write(rbi));
> +}
> +VMBUS_CHAN_ATTR_RO(write_avail);

This is upstream since a year.

But I wonder how this can work if vmbus_device_register is called,
and then something reads the populated sysfs files before vmbus_open returns.
Nothing protects rbi->ring_buffer in this case, which remains NULL
until vmbus_open populates it.

A simple reproduce, with a modular kernel, is to boot with init=/bin/bash
head /sys/bus/vmbus/devices/*/channels/*/*

Olaf

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