Re: [PATCH] drivers/cdrom: improved ioctl for media change detection

[Jens Axboe]

On 8/5/21 1:44 PM, Lukas Prediger wrote:
> The current implementation of the CDROM_MEDIA_CHANGED ioctl relies on
> global state, meaning that only one process can detect a disc change
> while the ioctl call will return 0 for other calling processes afterwards
> (see bug 213267 ).
> 
> This introduces a new cdrom ioctl, CDROM_TIMED_MEDIA_CHANGE, that
> works by maintaining a timestamp of the last detected disc change instead
> of a boolean flag: Processes calling this ioctl command can provide
> a timestamp of the last disc change known to them and receive
> an indication whether the disc was changed since then and the updated
> timestamp.
> 
> I considered fixing the buggy behavior in the original
> CDROM_MEDIA_CHANGED ioctl but that would require maintaining state
> for each calling process in the kernel, which seems like a worse
> solution than introducing this new ioctl.
> 
> Signed-off-by: Lukas Prediger <lumip@xxxxxxxx>
> ---
> This is my first patch sent to the kernel and I followed the recommended
> process as closely as I could. If I misstepped somewhere, please let me know.
> I also tried to find a mailing list more specific to the problem but was unable,
> please inform me if there is one where this should be sent instead.
> 
> Finally, I wasn't sure whether/how to add my name to the updated docs
> and if I should update the date on them (there have been some recent commits
> that did not result in credits given to the commit author or updating
> the dates in the docs, but those did not add any new content either), so
> info in the most appropriate way would be welcome.

Thanks for sending in the patch, appologies it's taken so long to get a
response. CDROM isn't really actively maintained much these days,
unfortunately. Should get a new maintainer.

Anyway, for this patch, few comments inline.


> diff --git a/Documentation/cdrom/cdrom-standard.rst b/Documentation/cdrom/cdrom-standard.rst
> index 5845960ca382..8b219ba9b427 100644
> --- a/Documentation/cdrom/cdrom-standard.rst
> +++ b/Documentation/cdrom/cdrom-standard.rst
> @@ -3,9 +3,10 @@ A Linux CD-ROM standard
>  =======================
>  
>  :Author: David van Leeuwen <david@xxxxxxxxxxxxxxxxxxx>
> -:Date: 12 March 1999
> +:Date: 5 August 2021
>  :Updated by: Erik Andersen (andersee@xxxxxxxxxx)
>  :Updated by: Jens Axboe (axboe@xxxxxxxx)
> +:Updated by: Lukas Prediger (lumip@xxxxxxxx)

Drop these changes, this is from a time from before git and revision
history, no need to update them further.

> diff --git a/Documentation/userspace-api/ioctl/cdrom.rst b/Documentation/userspace-api/ioctl/cdrom.rst
> index 3b4c0506de46..59305d04d034 100644
> --- a/Documentation/userspace-api/ioctl/cdrom.rst
> +++ b/Documentation/userspace-api/ioctl/cdrom.rst
> @@ -3,8 +3,9 @@ Summary of CDROM ioctl calls
>  ============================
>  
>  - Edward A. Falk <efalk@xxxxxxxxxx>
> +- Lukas Prediger <lumip@xxxxxxxx>
>  
> -November, 2004
> +August, 2021

Ditto

> diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
> index feb827eefd1a..af0721c96e06 100644
> --- a/drivers/cdrom/cdrom.c
> +++ b/drivers/cdrom/cdrom.c
> @@ -344,6 +344,12 @@ static void cdrom_sysctl_register(void);
>  
>  static LIST_HEAD(cdrom_list);
>  
> +static void signal_media_change(struct cdrom_device_info *cdi)
> +{
> +	cdi->mc_flags = 0x3; /* set media changed bits, on both queues */
> +	cdi->last_media_change_time = ktime_get_mono_fast_ns();
> +}

I'd just use jiffies for this, it's not really a case of something that
needs a fine grained clock source. That'll give you 1-10ms resolution,
which should be more than adequate for this. Then use jiffies_to_msecs()
and make the API be in miliseconds.

> @@ -2391,6 +2397,46 @@ static int cdrom_ioctl_media_changed(struct cdrom_device_info *cdi,
>  	return ret;
>  }
>  
> +/*
> + * Media change detection with timing information.
> + *
> + * arg is a pointer to a cdrom_timed_media_change_info struct.
> + * arg->last_media_change may be set by calling code to signal
> + * the timestamp of the last known media change (by the caller).
> + * Upon successful return, ioctl call will set arg->last_media_change
> + * to the latest media change timestamp known by the kernel/driver
> + * and set arg->has_changed to 1 if that timestamp is more recent
> + * than the timestamp set by the caller.
> + */
> +static int cdrom_ioctl_timed_media_change(struct cdrom_device_info *cdi,
> +		unsigned long arg)
> +{
> +	int ret;
> +	struct cdrom_timed_media_change_info __user *info;
> +	struct cdrom_timed_media_change_info tmp_info;
> +
> +	if (!CDROM_CAN(CDC_MEDIA_CHANGED))
> +		return -ENOSYS;
> +
> +	info = (struct cdrom_timed_media_change_info __user *)arg;
> +	cd_dbg(CD_DO_IOCTL, "entering CDROM_TIMED_MEDIA_CHANGE\n");
> +
> +	ret = cdrom_ioctl_media_changed(cdi, CDSL_CURRENT);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (copy_from_user(&tmp_info, info, sizeof(tmp_info)) != 0)
> +		return -EFAULT;
> +
> +	tmp_info.has_changed = (tmp_info.last_media_change < cdi->last_media_change_time);

If you make the jiffies change, you can use time_before() etc on these
which avoids the wrap.

> @@ -295,6 +297,19 @@ struct cdrom_generic_command
>  	};
>  };
>  
> +/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
> +struct cdrom_timed_media_change_info
> +{
> +	__u64   last_media_change;	/* Timestamp of the last detected media
> +					 * change. May be set by caller, updated
> +					 * upon successful return of ioctl.
> +					 */
> +	__u8    has_changed;		/* Set to 1 by ioctl if last detected media
> +					 * change was more recent than
> +					 * last_media_change set by caller.
> +					 */
> +};

The struct layout should be modified such that there are no holes or
padding in it. Probably just make the has_changed a flags thing, and
make it u64 as well. Then you can define bit 0 to be HAS_CHANGED, and
that leaves you room to add more flags in the future. Though the latter
probably isn't much of a concern here, but...

-- 
Jens Axboe