linux-kernel - Re: [PATCH] writeback: skip new or to-be-freed inodes

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Date:	Mon, 8 Jun 2009 19:07:26 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Eric Sandeen <sandeen@...deen.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...e.de>, Jeff Layton <jlayton@...hat.com>
Subject: Re: [PATCH] writeback: skip new or to-be-freed inodes

On Sat 06-06-09 11:07:25, Wu Fengguang wrote:
> 1) I_FREEING tests should be coupled with I_CLEAR
> 
> The two I_FREEING tests are racy because clear_inode() can set i_state to
> I_CLEAR between the clear of I_SYNC and the test of I_FREEING.
> 
> 2) skip I_WILL_FREE inodes in generic_sync_sb_inodes() to avoid possible
>    races with generic_forget_inode()
> 
> generic_forget_inode() sets I_WILL_FREE call writeback on its own, so
> generic_sync_sb_inodes() shall not try to step in and create possible races:
> 
>   generic_forget_inode
>     inode->i_state |= I_WILL_FREE;
>     spin_unlock(&inode_lock);
>                                        generic_sync_sb_inodes()
>                                          spin_lock(&inode_lock);
>                                          __iget(inode);
>                                          __writeback_single_inode
>                                            // see non zero i_count
>  may WARN here ==>                         WARN_ON(inode->i_state & I_WILL_FREE);
>                                          spin_unlock(&inode_lock);
>  may call generic_forget_inode again ==> iput(inode);
> 
> The above race and warning didn't turn up because writeback_inodes() holds
> the s_umount lock, so generic_forget_inode() finds MS_ACTIVE and returns
> early. But we are not sure the UBIFS calls and future callers will guarantee
> that. So skip I_WILL_FREE inodes for the sake of safety.
  OK, the patch looks fine. Acked-by: Jan Kara <jack@...e.cz>

									Honza 
> CC: Jan Kara <jack@...e.cz>
> CC: Eric Sandeen <sandeen@...deen.net>
> Acked-by: Jeff Layton <jlayton@...hat.com>
> CC: Masayoshi MIZUMA <m.mizuma@...fujitsu.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  fs/fs-writeback.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> --- linux.orig/fs/fs-writeback.c
> +++ linux/fs/fs-writeback.c
> @@ -316,7 +316,7 @@ __sync_single_inode(struct inode *inode,
>  	spin_lock(&inode_lock);
>  	WARN_ON(inode->i_state & I_NEW);
>  	inode->i_state &= ~I_SYNC;
> -	if (!(inode->i_state & I_FREEING)) {
> +	if (!(inode->i_state & (I_FREEING | I_CLEAR))) {
>  		if (!(inode->i_state & I_DIRTY) &&
>  		    mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
>  			/*
> @@ -487,7 +487,7 @@ void generic_sync_sb_inodes(struct super
>  			break;
>  		}
>  
> -		if (inode->i_state & I_NEW) {
> +		if (inode->i_state & (I_NEW | I_WILL_FREE)) {
>  			requeue_io(inode);
>  			continue;
>  		}
> @@ -518,7 +518,7 @@ void generic_sync_sb_inodes(struct super
>  		if (current_is_pdflush() && !writeback_acquire(bdi))
>  			break;
>  
> -		BUG_ON(inode->i_state & I_FREEING);
> +		BUG_ON(inode->i_state & (I_FREEING | I_CLEAR));
>  		__iget(inode);
>  		pages_skipped = wbc->pages_skipped;
>  		__writeback_single_inode(inode, wbc);
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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