linux-kernel - Re: [PATCHv2 09/10] vfio/mdev: Avoid creating sysfs remove file on stale device removal

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Date:   Wed, 8 May 2019 19:16:35 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kwankhede@...dia.com, alex.williamson@...hat.com, cjia@...dia.com
Subject: Re: [PATCHv2 09/10] vfio/mdev: Avoid creating sysfs remove file on
 stale device removal

On Tue, 30 Apr 2019 17:49:36 -0500
Parav Pandit <parav@...lanox.com> wrote:

> If device is removal is initiated by two threads as below, mdev core
> attempts to create a syfs remove file on stale device.
> During this flow, below [1] call trace is observed.
> 
>      cpu-0                                    cpu-1
>      -----                                    -----
>   mdev_unregister_device()
>     device_for_each_child
>        mdev_device_remove_cb
>           mdev_device_remove
>                                        user_syscall
>                                          remove_store()
>                                            mdev_device_remove()
>                                         [..]
>    unregister device();
>                                        /* not found in list or
>                                         * active=false.
>                                         */
>                                           sysfs_create_file()
>                                           ..Call trace
> 
> Now that mdev core follows correct device removal system of the linux
> bus model, remove shouldn't fail in normal cases. If it fails, there is
> no point of creating a stale file or checking for specific error status.

Which error cases are left? Is there anything that does not indicate
that something got terribly messed up internally?

> 
> kernel: WARNING: CPU: 2 PID: 9348 at fs/sysfs/file.c:327
> sysfs_create_file_ns+0x7f/0x90
> kernel: CPU: 2 PID: 9348 Comm: bash Kdump: loaded Not tainted
> 5.1.0-rc6-vdevbus+ #6
> kernel: Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b
> 08/09/2016
> kernel: RIP: 0010:sysfs_create_file_ns+0x7f/0x90
> kernel: Call Trace:
> kernel: remove_store+0xdc/0x100 [mdev]
> kernel: kernfs_fop_write+0x113/0x1a0
> kernel: vfs_write+0xad/0x1b0
> kernel: ksys_write+0x5a/0xe0
> kernel: do_syscall_64+0x5a/0x210
> kernel: entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Signed-off-by: Parav Pandit <parav@...lanox.com>
> ---
>  drivers/vfio/mdev/mdev_sysfs.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/mdev/mdev_sysfs.c b/drivers/vfio/mdev/mdev_sysfs.c
> index 9f774b91d275..ffa3dcebf201 100644
> --- a/drivers/vfio/mdev/mdev_sysfs.c
> +++ b/drivers/vfio/mdev/mdev_sysfs.c
> @@ -237,10 +237,8 @@ static ssize_t remove_store(struct device *dev, struct device_attribute *attr,
>  		int ret;
>  
>  		ret = mdev_device_remove(dev);
> -		if (ret) {
> -			device_create_file(dev, attr);
> +		if (ret)

Should you merge this into the previous patch?

>  			return ret;
> -		}
>  	}
>  
>  	return count;