From: Naman Jain <namj...@linux.microsoft.com> Sent: Thursday, March 27, 2025
10:28 PM
>
> On regular bootup, devices get registered to VMBus first, so when
> uio_hv_generic driver for a particular device type is probed,
> the device is already initialized and added, so sysfs creation in
> uio_hv_generic probe works fine. However, when device is removed
> and brought back, the channel rescinds and device again gets
> registered to VMBus. However this time, the uio_hv_generic driver is
> already registered to probe for that device and in this case sysfs
> creation is tried before the device's kobject gets initialized
> completely.
>
> Fix this by moving the core logic of sysfs creation for ring buffer,
> from uio_hv_generic to HyperV's VMBus driver, where rest of the sysfs
> attributes for the channels are defined. While doing that, make use
> of attribute groups and macros, instead of creating sysfs directly,
> to ensure better error handling and code flow.
>
> Problem path:
> vmbus_process_offer (new offer comes for the VMBus device)
> vmbus_add_channel_work
> vmbus_device_register
> |-> device_register
> | |...
> | |-> hv_uio_probe
> | |...
> | |-> sysfs_create_bin_file (leads to a warning as
> | primary channel's kobject, which is used to
> | create sysfs is not yet initialized)
> |-> kset_create_and_add
> |-> vmbus_add_channel_kobj (initialization of primary channel's
> kobject happens later)
>
> Above code flow is sequential and the warning is always reproducible in
> this path.
>
> Fixes: 9ab877a6ccf8 ("uio_hv_generic: make ring buffer attribute for primary
> channel")
> Cc: sta...@kernel.org
> Suggested-by: Saurabh Sengar <ssen...@linux.microsoft.com>
> Suggested-by: Michael Kelley <mhkli...@outlook.com>
> Signed-off-by: Naman Jain <namj...@linux.microsoft.com>
> ---
> drivers/hv/hyperv_vmbus.h | 6 ++
> drivers/hv/vmbus_drv.c | 110 ++++++++++++++++++++++++++++++++++-
> drivers/uio/uio_hv_generic.c | 33 +++++------
> include/linux/hyperv.h | 6 ++
> 4 files changed, 134 insertions(+), 21 deletions(-)
>
[snip]
> +/**
> + * hv_create_ring_sysfs() - create "ring" sysfs entry corresponding to ring
> buffers for a channel.
> + * @channel: Pointer to vmbus_channel structure
> + * @hv_mmap_ring_buffer: function pointer for initializing the function to
> be called on mmap of
> + * channel's "ring" sysfs node, which is for the ring
> buffer of that channel.
> + * Function pointer is of below type:
> + * int (*hv_mmap_ring_buffer)(struct vmbus_channel
> *channel,
> + * struct vm_area_struct
> *vma))
> + * This has a pointer to the channel and a pointer to
> vm_area_struct,
> + * used for mmap, as arguments.
> + *
> + * Sysfs node for ring buffer of a channel is created along with other
> fields, however its
> + * visibility is disabled by default. Sysfs creation needs to be controlled
> when the use-case
> + * is running.
> + * For example, HV_NIC device is used either by uio_hv_generic or hv_netvsc
> at any given point of
> + * time, and "ring" sysfs is needed only when uio_hv_generic is bound to
> that device. To avoid
> + * exposing the ring buffer by default, this function is reponsible to
> enable visibility of
> + * ring for userspace to use.
> + * Note: Race conditions can happen with userspace and it is not encouraged
> to create new
> + * use-cases for this. This was added to maintain backward compatibility,
> while solving
> + * one of the race conditions in uio_hv_generic while creating sysfs.
> + *
> + * Returns 0 on success or error code on failure.
> + */
> +int hv_create_ring_sysfs(struct vmbus_channel *channel,
> + int (*hv_mmap_ring_buffer)(struct vmbus_channel
> *channel,
> + struct vm_area_struct *vma))
> +{
> + struct kobject *kobj = &channel->kobj;
> + struct vmbus_channel *primary_channel = channel->primary_channel ?
> + channel->primary_channel : channel;
> +
> + channel->mmap_ring_buffer = hv_mmap_ring_buffer;
> + channel->ring_sysfs_visible = true;
> +
> + /*
> + * Skip updating the sysfs group if the primary channel is not yet
> initialized and sysfs
> + * group is not yet created. In those cases, the 'ring' will be created
> later in
> + * vmbus_device_register() -> vmbus_add_channel_kobj().
> + */
> + if (!primary_channel->device_obj->channels_kset)
> + return 0;
This test doesn't accomplish what you want. It tests if the "channels" directory
has been created, but not if the numbered subdirectory for this channel has been
created. sysfs_update_group() operates on the numbered subdirectory and
could still fail because it hasn't been created yet.
My recommendation is to not try to do a test, and just let sysfs_update_group()
fail in that case (and ignore the error).
Michael
> +
> + return sysfs_update_group(kobj, &vmbus_chan_group);
> +}
> +EXPORT_SYMBOL_GPL(hv_create_ring_sysfs);
