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    SubjectRe: ext3 lockdep warning in 2.6.25-rc6
    On Tue 01-04-08 17:23:34, Erez Zadok wrote:
    <snip>
    > > From f5e41087e345fa5c3b46ac36e6e4a654d2f7f624 Mon Sep 17 00:00:00 2001
    > > From: Jan Kara <jack@suse.cz>
    > > Date: Tue, 18 Mar 2008 14:38:06 +0100
    > > Subject: [PATCH] Fix drop_pagecache_sb() to not call __invalidate_mapping_pages() under
    > > inode_lock.
    > >
    > > Signed-off-by: Jan Kara <jack@suse.cz>
    > > ---
    > > fs/drop_caches.c | 8 +++++++-
    > > 1 files changed, 7 insertions(+), 1 deletions(-)
    > >
    > > diff --git a/fs/drop_caches.c b/fs/drop_caches.c
    > > index 59375ef..f5aae26 100644
    > > --- a/fs/drop_caches.c
    > > +++ b/fs/drop_caches.c
    > > @@ -14,15 +14,21 @@ int sysctl_drop_caches;
    > >
    > > static void drop_pagecache_sb(struct super_block *sb)
    > > {
    > > - struct inode *inode;
    > > + struct inode *inode, *toput_inode = NULL;
    > >
    > > spin_lock(&inode_lock);
    > > list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
    > > if (inode->i_state & (I_FREEING|I_WILL_FREE))
    > > continue;
    > > + __iget(inode);
    > > + spin_unlock(&inode_lock);
    > > __invalidate_mapping_pages(inode->i_mapping, 0, -1, true);
    > > + iput(toput_inode);
    > > + toput_inode = inode;
    > > + spin_lock(&inode_lock);
    > > }
    > > spin_unlock(&inode_lock);
    > > + iput(toput_inode);
    > > }
    >
    > Jan, I'll be happy to test this, but I don't understand two things about
    > this patch:
    >
    > 1. Is it safe to unlock and re-lock inode_lock temporarily within the loop?
    >
    > 2. What's the motivation behind having the second toput_inode pointer? It
    > appears that the first iteration through the loop, toput_inode will be
    > NULL, so we'll be iput'ing a NULL pointer (which is ok). So you're
    > trying to iput the previous inode pointer that the list iterated over,
    > right? Is that intended?
    I'll try to explain the locking here:
    1) We are not allowed to call into __invalidate_mapping_pages() with
    inode_lock held - that it the bug lockdep is complaining about. Moreover it
    leads to rather long waiting times for inode_lock (quite possibly several
    seconds).
    2) When we release inode_lock, we need to protect from inode going away,
    thus we hold reference to it - that guarantees us inode stays in the list.
    3) When we continue scanning of the list we must get inode_lock before we
    put the inode reference to avoid races. But we cannot do iput() when we
    hold inode_lock. Thus we save pointer to inode and do iput() next time we
    have released the inode_lock...

    > Peter's post:
    >
    > http://lkml.org/lkml/2008/3/23/202
    >
    > includes a reference to a mail by Andrew which implies that the fix may be
    > much more involved than what you outlined above, no?
    Definitely we can do a more involved fix ;) What Andrew proposes would have
    some other benefits as well. But until somebody gets to that, this slight
    hack should work fine (Andrew has already merged my patch in -mm so I
    guess he agrees ;).

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


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