linux-kernel - Re: [PATCH 01/19] char_dev: replace cdev

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Date:   Thu, 27 Aug 2020 09:25:07 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Denis Efremov <efremov@...ux.com>,
        "David S. Miller" <davem@...emloft.net>,
        Song Liu <song@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, linux-raid@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH 01/19] char_dev: replace cdev_map with an xarray

On 8/26/20 8:24 AM, Christoph Hellwig wrote:
> None of the complicated overlapping regions bits of the kobj_map are
> required for the character device lookup, so just a trivial xarray
> instead.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  fs/char_dev.c | 94 +++++++++++++++++++++++++--------------------------
>  fs/dcache.c   |  1 -
>  fs/internal.h |  5 ---
>  3 files changed, 46 insertions(+), 54 deletions(-)
> 
> diff --git a/fs/char_dev.c b/fs/char_dev.c
> index ba0ded7842a779..6c4d6c4938f14b 100644
> --- a/fs/char_dev.c
> +++ b/fs/char_dev.c
> @@ -17,7 +17,6 @@
>  #include <linux/seq_file.h>
>  
>  #include <linux/kobject.h>
> -#include <linux/kobj_map.h>
>  #include <linux/cdev.h>
>  #include <linux/mutex.h>
>  #include <linux/backing-dev.h>
> @@ -25,8 +24,7 @@
>  
>  #include "internal.h"
>  
> -static struct kobj_map *cdev_map;
> -
> +static DEFINE_XARRAY(cdev_map);
>  static DEFINE_MUTEX(chrdevs_lock);
>  
>  #define CHRDEV_MAJOR_HASH_SIZE 255
> @@ -367,6 +365,29 @@ void cdev_put(struct cdev *p)
>  	}
>  }
>  
> +static struct cdev *cdev_lookup(dev_t dev)
> +{
> +	struct cdev *cdev;
> +
> +retry:
> +	mutex_lock(&chrdevs_lock);
> +	cdev = xa_load(&cdev_map, dev);
> +	if (!cdev) {
> +		mutex_unlock(&chrdevs_lock);
> +
> +		if (request_module("char-major-%d-%d",
> +				   MAJOR(dev), MINOR(dev)) > 0)
> +			/* Make old-style 2.4 aliases work */
> +			request_module("char-major-%d", MAJOR(dev));
> +		goto retry;
> +	}
> +
> +	if (!cdev_get(cdev))
> +		cdev = NULL;
> +	mutex_unlock(&chrdevs_lock);
> +	return cdev;
> +}
> +
>  /*
>   * Called every time a character special file is opened
>   */
> @@ -380,13 +401,10 @@ static int chrdev_open(struct inode *inode, struct file *filp)
>  	spin_lock(&cdev_lock);
>  	p = inode->i_cdev;
>  	if (!p) {
> -		struct kobject *kobj;
> -		int idx;
>  		spin_unlock(&cdev_lock);
> -		kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
> -		if (!kobj)
> +		new = cdev_lookup(inode->i_rdev);
> +		if (!new)
>  			return -ENXIO;
> -		new = container_of(kobj, struct cdev, kobj);
>  		spin_lock(&cdev_lock);
>  		/* Check i_cdev again in case somebody beat us to it while
>  		   we dropped the lock. */
> @@ -454,18 +472,6 @@ const struct file_operations def_chr_fops = {
>  	.llseek = noop_llseek,
>  };
>  
> -static struct kobject *exact_match(dev_t dev, int *part, void *data)
> -{
> -	struct cdev *p = data;
> -	return &p->kobj;
> -}
> -
> -static int exact_lock(dev_t dev, void *data)
> -{
> -	struct cdev *p = data;
> -	return cdev_get(p) ? 0 : -1;
> -}
> -
>  /**
>   * cdev_add() - add a char device to the system
>   * @p: the cdev structure for the device
> @@ -478,7 +484,7 @@ static int exact_lock(dev_t dev, void *data)
>   */
>  int cdev_add(struct cdev *p, dev_t dev, unsigned count)
>  {
> -	int error;
> +	int error, i;
>  
>  	p->dev = dev;
>  	p->count = count;
> @@ -486,14 +492,22 @@ int cdev_add(struct cdev *p, dev_t dev, unsigned count)
>  	if (WARN_ON(dev == WHITEOUT_DEV))
>  		return -EBUSY;
>  
> -	error = kobj_map(cdev_map, dev, count, NULL,
> -			 exact_match, exact_lock, p);
> -	if (error)
> -		return error;
> +	mutex_lock(&chrdevs_lock);
> +	for (i = 0; i < count; i++) {
> +		error = xa_insert(&cdev_map, dev + i, p, GFP_KERNEL);
> +		if (error)
> +			goto out_unwind;
> +	}
> +	mutex_unlock(&chrdevs_lock);
>  
>  	kobject_get(p->kobj.parent);
> -
>  	return 0;
> +
> +out_unwind:
> +	while (--i >= 0)
> +		xa_erase(&cdev_map, dev + i);
> +	mutex_unlock(&chrdevs_lock);
> +	return error;
>  }
>  
>  /**
Do you really need the mutex?
Wouldn't xa_store_range() be better and avoid the mutex?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@...e.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer