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

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Date:	Sat, 6 Jun 2009 11:07:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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: [PATCH] writeback: skip new or to-be-freed inodes

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.

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);
--
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