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    SubjectRe: [PATCH] skip I_CLEAR state inodes
    On Wed 03-06-09 22:10:21, Wu Fengguang wrote:
    > On Tue, Jun 02, 2009 at 07:37:36PM +0800, 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?
    > > >
    > > > > 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@redhat.com>
    > > > Reported-by: Eric Sandeen <sandeen@sandeen.net>
    > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.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)) {
    > > 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.
    > >
    > > > /*
    > > > @@ -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);
    > > Looking at this code, it looks a bit suspicious. What prevents this s_io
    > > list scan to race with inode freeing? In particular generic_forget_inode()
    >
    > Good catch.
    >
    > > can drop inode_lock to write the inode and in the mean time
    > > generic_sync_sb_inodes() can come, get a reference to the inode and start
    > > it's writeback... Subsequent iput() would then call generic_forget_inode()
    >
    > Another possibility:
    >
    > 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
    > WARN_ON(inode->i_state & I_WILL_FREE);
    >
    > I'm wondering why didn't we saw reports on the last WARN_ON()?
    > Did we missed something?
    I meant the above race in my description ;-). Anyway, the race can happen
    only if we are unmounting the filesystem (normally, we bail out on
    sb->s_flags & MS_ACTIVE check - yes, it's a bit hidden and it also took me
    a while to understand why we weren't seeing tons of warnings...).

    > > on the inode again. So shouldn't we skip I_FREEING|I_CLEAR|I_WILL_FREE|I_NEW
    > > inodes in this scan like we do for later in the function for another scan?

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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