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

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Message-ID: <CY4PR21MB0773D6885EC71F92D1A1369FD7F80@CY4PR21MB0773.namprd21.prod.outlook.com>
Date:   Thu, 18 Oct 2018 15:32:35 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Olaf Hering <olaf@...fle.de>,
        Stephen Hemminger <stephen@...workplumber.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>
Subject: RE: [PATCH V2 3/4] vmbus: add per-channel sysfs info

>From Olaf Hering  Sent: Thursday, October 18, 2018 8:20 AM
>
> > 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/*/*
> 

There are multiple race conditions with this and other VMbus sysfs information.
There's a race on the close path as well.  I've got an action on my list to get it
cleaned up.

Michael