linux-kernel - Re: [PATCH] skip I

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Date:	Tue, 2 Jun 2009 16:55:23 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	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>
Subject: Re: [PATCH] skip I_CLEAR state inodes

On Tue, Jun 02, 2009 at 05:38:35AM +0800, Eric Sandeen wrote:
> Wu Fengguang wrote:
> > Add I_CLEAR tests to drop_pagecache_sb(), generic_sync_sb_inodes() and
> > add_dquot_ref().
> > 
> > clear_inode() will switch inode state from I_FREEING to I_CLEAR,
> > and do so _outside_ of inode_lock. So any I_FREEING testing is
> > incomplete without the testing of I_CLEAR.
> > 
> > Masayoshi MIZUMA first discovered the bug in drop_pagecache_sb() and
> > Jan Kara reminds fixing the other two cases. Thanks!
> 
> Is there a reason it's not done for __sync_single_inode as well?

It missed the glance because it don't have an obvious '|' in the line ;)

> Jeff Layton asked the question and I'm following it up :)
> 
> __sync_single_inode currently only tests I_FREEING, but I think we are
> safe because __sync_single_inode sets I_SYNC, and clear_inode waits for
> I_SYNC to be cleared before it changes I_STATE.

But I_SYNC is removed just before the I_FREEING test, so we still have
a small race window?

> On the other hand, testing I_CLEAR here probably would be safe anyway,
> and it'd be bonus points for consistency?

So let's add the I_CLEAR test?

> Same basic question for generic_sync_sb_inodes, which has a
> BUG_ON(inode->i_state & I_FREEING), seems like this could check I_CLWAR
> as well?

Yes, we can add I_CLEAR here to catch more error condition.

Thanks,
Fengguang

---
skip I_CLEAR state inodes in writeback routines

The I_FREEING test in __sync_single_inode() is racy because
clear_inode() can set i_state to I_CLEAR between the clear of I_SYNC
and the test of I_FREEING.

Also extend the coverage of BUG_ON(I_FREEING) to I_CLEAR.

Reported-by: Jeff Layton <jlayton@...hat.com>
Reported-by: Eric Sandeen <sandeen@...deen.net>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 fs/fs-writeback.c |    4 ++--
 1 file changed, 2 insertions(+), 2 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)) {
 			/*
@@ -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);
--
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