linux-kernel - Re: [PATCH] skip I

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Message-ID: <4A259E49.4020003@sandeen.net>
Date:	Tue, 02 Jun 2009 16:48:57 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Jan Kara <jack@...e.cz>
CC:	Wu Fengguang <fengguang.wu@...el.com>,
	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>
Subject: Re: [PATCH] skip I_CLEAR state inodes

Jan Kara wrote:
> On Tue 02-06-09 16:55:23, Wu Fengguang wrote:
>> 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?

yep that's right.

        inode->i_state &= ~I_SYNC;
	>>> clear_inode->inode_sync_wait here and find it clear <<<
        if (!(inode->i_state & I_FREEING)) {

...

>> --- 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)) {

>   Is the whole if needed? I had an impression that everyone calling
> __sync_single_inode() should better take care it does not race with inode
> freeing... So WARN_ON would be more appropriate IMHO.

Maybe both then (both a WARN on and then the test (defensive here, I
guess)) because if we continue we may wander into a poisoned list
pointer and explode, right?

-Eric
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