Re: [PATCH, RFC] prune back iprune_sem

[Jan Kara <jack@suse.cz>]

  Hi,

On Tue 15-02-11 11:29:16, Christoph Hellwig wrote:
> On Tue, Nov 02, 2010 at 07:45:36PM +0100, Christoph Hellwig wrote:
> > iprune_sem is continously giving us lockdep warnings because we do take it in
> > read mode in the reclaim path, but we're also doing non-NOFS allocations under
> > it taken in write mode.
> > 
> > Taking a bit deeper look at it I think it's fixable quite trivially:
> > 
> >  - for invalidate_inodes we do not need iprune_sem at all.  We have an
> >  active reference on the superblock, so the filesystem is not going
> >  away until it has finished.
> >  - for evict_inodes we do need it, to make sure prune_icache has done
> >  it's work before we tear down the superblock.  But there is no reason
> >  to hold it over the actual reclaim operation - it's enough to cycle
> >  through it after the actual reclaim to make sure we wait for any
> >  pending prune_icache to complete.
  I just wonder: So with this change, evict_inodes() can start seeing
inodes, that are just being freed by prune_icache(). Thus we can trigger
WARN_ON() in evict_inodes():
                if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
                        WARN_ON(1);
                        continue;
                }
  Otherwise, the change looks safe to me. BTW, the iprune_sem is now used
only so that evict_inodes() can wait for prune_icache() to finish so maybe
we could have something simpler for that?

								Honza
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > 
> > diff --git a/fs/inode.c b/fs/inode.c
> > index ae2727a..cfa7722 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -492,8 +492,6 @@ void evict_inodes(struct super_block *sb)
> >  	struct inode *inode, *next;
> >  	LIST_HEAD(dispose);
> >  
> > -	down_write(&iprune_sem);
> > -
> >  	spin_lock(&inode_lock);
> >  	list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
> >  		if (atomic_read(&inode->i_count))
> > @@ -518,6 +516,13 @@ void evict_inodes(struct super_block *sb)
> >  	spin_unlock(&inode_lock);
> >  
> >  	dispose_list(&dispose);
> > +
> > +	/*
> > +	 * Cycle through iprune_sem to make sure any inode that prune_icache
> > +	 * moved off the list before we took the lock has been fully torn
> > +	 * down.
> > +	 */
> > +	down_write(&iprune_sem);
> >  	up_write(&iprune_sem);
> >  }
> >  
> > @@ -534,8 +539,6 @@ int invalidate_inodes(struct super_block *sb)
> >  	struct inode *inode, *next;
> >  	LIST_HEAD(dispose);
> >  
> > -	down_write(&iprune_sem);
> > -
> >  	spin_lock(&inode_lock);
> >  	list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
> >  		if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE))
> > @@ -559,7 +562,6 @@ int invalidate_inodes(struct super_block *sb)
> >  	spin_unlock(&inode_lock);
> >  
> >  	dispose_list(&dispose);
> > -	up_write(&iprune_sem);
> >  
> >  	return busy;
> >  }
> ---end quoted text---
> --
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR