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From:   Michael Kelley <mikelley@microsoft.com>
To:     Olaf Hering <olaf@aepfle.de>,
        Stephen Hemminger <stephen@networkplumber.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>
Subject: RE: [PATCH V2 3/4] vmbus: add per-channel sysfs info
Thread-Topic: [PATCH V2 3/4] vmbus: add per-channel sysfs info
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Date:   Thu, 18 Oct 2018 15:32:35 +0000
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From Olaf Hering  Sent: Thursday, October 18, 2018 8:20 AM
>
> > This extends existing vmbus related sysfs structure to provide per-chan=
nel
> > state information. This is useful when diagnosing issues with multiple
> > queues in networking and storage.
>=20
> > +++ b/drivers/hv/vmbus_drv.c
> > +static ssize_t write_avail_show(const struct vmbus_channel *channel, c=
har *buf)
> > +{
> > +	const struct hv_ring_buffer_info *rbi =3D &channel->outbound;
> > +
> > +	return sprintf(buf, "%u\n", hv_get_bytes_to_write(rbi));
> > +}
> > +VMBUS_CHAN_ATTR_RO(write_avail);
>=20
> This is upstream since a year.
>=20
> But I wonder how this can work if vmbus_device_register is called,
> and then something reads the populated sysfs files before vmbus_open retu=
rns.
> Nothing protects rbi->ring_buffer in this case, which remains NULL
> until vmbus_open populates it.
>=20
> A simple reproduce, with a modular kernel, is to boot with init=3D/bin/ba=
sh
> head /sys/bus/vmbus/devices/*/channels/*/*
>=20

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

Michael