Re: [PATCH] Btrfs: use rcu to protect device->name V2

[Zach Brown <zab@zabbo.net>]

> -		if (state->print_mask&  BTRFSIC_PRINT_MASK_SUPERBLOCK_WRITE)
> +		if (state->print_mask&  BTRFSIC_PRINT_MASK_SUPERBLOCK_WRITE) {
> +			struct rcu_string *name;
> +
> +			rcu_read_lock();
> +			name = rcu_dereference(device->name);
>   			printk(KERN_INFO "New initial S-block (bdev %p, %s)"
>   			       " @%llu (%s/%llu/%d)\n",
> -			       superblock_bdev, device->name,
> +			       superblock_bdev, name->str,
>   			       (unsigned long long)dev_bytenr,
>   			       dev_state->name,
>   			       (unsigned long long)dev_bytenr,
>   			       superblock_mirror_num);
> +			rcu_read_unlock();
> +		}

That's a whole lot of noise at every call site.  How about some helpers?
In sloppy-code..

#define printk_in_rcu(args) do { rcu; printk; rcu; } while (0)

#define device_name_rcu(dev) ({
	struct rcu_string *name = rcu_dereference(dev->name);
	name->str; )}

	printk_in_rcu("HELLO FRIENDS %s", device_name_rcu(dev));

It's kind of annoying to have a wrapper for printk(), pr_debug(), etc,
but it sure seems better than implementing the rcu consistency rules
around every output.  (And multiple rcu_string locals for multiple
device names..)

- z