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    SubjectRe: [PATCH] skip I_CLEAR state inodes
    On Tue, 02 Jun 2009 16:48:57 -0500
    Eric Sandeen <sandeen@sandeen.net> wrote:

    > 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?
    >

    Right.

    I think removing this check would be roughly equivalent to adding a
    BUG_ON. It just wouldn't reliably pop since it would depend on the
    timing of the race.

    Keeping the check and adding a WARN seems like a reasonable thing to
    do. Maybe after a few releases if no one has seen the WARN fire we can
    look at removing the check (or maybe turn it into a BUG)?

    --
    Jeff Layton <jlayton@redhat.com>


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