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@xxxxxxxxxx>
> > Reported-by: Eric Sandeen <sandeen@xxxxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > ---
> > 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?
> 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?
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