Re: [PATCH 00/11] [GIT PULL] more updates for the tag format

[Theodore Tso <tytso@mit.edu>]

On Thu, Jun 11, 2009 at 07:14:36PM +0200, Frederic Weisbecker wrote:
> 
> For the filters, we could enter the text name which would be
> internally converted into a dev_t, there should be no problem.
> 
> Also the raw dev_t can be stored and then human-friendly printed on
> print time.
> 
> Both seem about trivial to add.

If someone wants to take this code and drop it into the core tracing
code, please feel free.

						- Ted

/* 
 * jbd2_dev_to_name is a utility function used by the jbd2 and ext4 
 * tracing infrastructure to map a dev_t to a device name.
 *
 * The caller should use rcu_read_lock() in order to make sure the
 * device name stays valid until its done with it.  We use
 * rcu_read_lock() as well to make sure we're safe in case the caller
 * gets sloppy, and because rcu_read_lock() is cheap and can be safely
 * nested.
 */
struct devname_cache {
	struct rcu_head	rcu;
	dev_t		device;
	char		devname[BDEVNAME_SIZE];
};
#define CACHE_SIZE_BITS 6
static struct devname_cache *devcache[1 << CACHE_SIZE_BITS];
static DEFINE_SPINLOCK(devname_cache_lock);

static void free_devcache(struct rcu_head *rcu)
{
	kfree(rcu);
}

const char *jbd2_dev_to_name(dev_t device)
{
	int	i = hash_32(device, CACHE_SIZE_BITS);
	char	*ret;
	struct block_device *bd;

	rcu_read_lock();
	if (devcache[i] && devcache[i]->device == device) {
		ret = devcache[i]->devname;
		rcu_read_unlock();
		return ret;
	}
	rcu_read_unlock();

	spin_lock(&devname_cache_lock);
	if (devcache[i]) {
		if (devcache[i]->device == device) {
			ret = devcache[i]->devname;
			spin_unlock(&devname_cache_lock);
			return ret;
		}
		call_rcu(&devcache[i]->rcu, free_devcache);
	}
	devcache[i] = kmalloc(sizeof(struct devname_cache), GFP_KERNEL);
	if (!devcache[i]) {
		spin_unlock(&devname_cache_lock);
		return "NODEV-ALLOCFAILURE"; /* Something non-NULL */
	}
	devcache[i]->device = device;
	bd = bdget(device);
	if (bd) {
		bdevname(bd, devcache[i]->devname);
		bdput(bd);
	} else
		__bdevname(device, devcache[i]->devname);
	ret = devcache[i]->devname;
	spin_unlock(&devname_cache_lock);
	return ret;
}
EXPORT_SYMBOL(jbd2_dev_to_name);