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

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Message-ID: <20200121014924.GI8904@ZenIV.linux.org.uk>
Date:   Tue, 21 Jan 2020 01:49:24 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Namjae Jeon <linkinjeon@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, valdis.kletnieks@...edu, hch@....de,
        sj1557.seo@...sung.com, pali.rohar@...il.com, arnd@...db.de,
        namjae.jeon@...sung.com
Subject: Re: [PATCH v12 02/13] exfat: add super block operations

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();