Re: [PATCH v12 02/13] exfat: add super block operations

Al Viro
2020-01-21T01:49:29Z

On Mon, Jan 20, 2020 at 09:44:17PM +0900, Namjae Jeon wrote:

> +static void exfat_put_super(struct super_block *sb)
> +{
> + struct exfat_sb_info *sbi = EXFAT_SB(sb);
> +
> + mutex_lock(&sbi->s_lock);
> + if (test_and_clear_bit(EXFAT_SB_DIRTY, &sbi->s_state))
> + sync_blockdev(sb->s_bdev);
> + exfat_set_vol_flags(sb, VOL_CLEAN);
> + exfat_free_upcase_table(sb);
> + exfat_free_bitmap(sb);
> + mutex_unlock(&sbi->s_lock);
> +
> + if (sbi->nls_io) {
> + unload_nls(sbi->nls_io);
> + sbi->nls_io = NULL;
> + }
> + exfat_free_iocharset(sbi);
> + sb->s_fs_info = NULL;
> + kfree(sbi);
> +}

You need to RCU-delay freeing sbi and zeroing ->nls_io. *Everything*
used by ->d_compare() and ->d_hash() needs that treatment. RCU-mode
pathwalk can stray into a filesystem that has already been lazy-umounted
and is just one close() away from shutdown. It's OK, as long as you
make sure that all structures used in methods that could be called
in RCU mode (->d_compare(), ->d_hash(), rcu-case ->d_revalidate(),
rcu-case ->permission()) have destruction RCU-delayed. Look at
what VFAT is doing; that's precisely the reason for that delayed_free()
thing in there.

> +static void exfat_destroy_inode(struct inode *inode)
> +{
> + kmem_cache_free(exfat_inode_cachep, EXFAT_I(inode));
> +}

No. Again, that MUST be RCU-delayed; either put an explicit
call_rcu() here, or leave as-is, but make that ->free_inode().

> +static void __exit exit_exfat_fs(void)
> +{
> + kmem_cache_destroy(exfat_inode_cachep);
> + unregister_filesystem(&exfat_fs_type);

... and add rcu_barrier() here.

> + exfat_cache_shutdown();