Re: [RFC][PATCH] fs: Prevent syncing frozen file system

[Jan Kara]

On Fri 10-07-15 09:40:12, Dave Chinner wrote:
> On Thu, Jul 09, 2015 at 07:45:45PM +0200, Lukas Czerner wrote:
> > Currently we can end up in a deadlock because of broken
> > sb_start_write -> s_umount ordering.
> > 
> > The race goes like this:
> > 
> >  - write the file
> >  - unlink the file - final_iput will not be calles as file is opened
> >  - freeze the file system
> >  - Now simultaneously close the file and call sync (or syncfs on that
> >    particular file system). Sync will get to wait_sb_inodes() where it will
> >    grab the referece to the inode (__iget()) and later to call iput().
> 
> This problem goes away with the sync scalability patchset that josef
> has been trying to get merged:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next.git superblock-scaling
> 
> That patchset removes the full sb inodes list walk in
> wait_sb_inodes() and replaces it with a walk of inodes cleaned
> during the sync, which will be an empty list in the case of sync
> running on an empty filesystem. This commit does the work:
> 
> https://git.kernel.org/cgit/linux/kernel/git/josef/btrfs-next.git/commit/?h=superblock-scaling&id=9bea30d5f4521db674203f365b1e0970588b2650
> 
> <As a separate note, can we *please* get that patchset merged given
> that there are now several outstanding issues that it fixes in one
> go?>

Not sure where that got stuck - oh, maybe on Tejun's memcg writeback series
which was clashing with it. Josef?

> >    If we manage to close the file and drop the reference in between those
> >    calls sync will attempt to do a iput_final() because the inode is now
> >    unlinked and we're holding the last reference to it. This will
> >    however block on a frozen file system (ext4_delete_inode for
> >    example).
> > 
> > Note that I've not been able to reproduce the issue, I've only seen this
> > happen once. However with some instrumentation (like msleep() in the
> > wait_sb_inodes() it can be achieved.
> > 
> > Fix this by properly doing sb_start_write/sb_end_write to prevent us
> > from fsfreeze.
> > 
> > Note that with this patch syncfs will block on the frozen file system
> > which is probably ok, but sync will block if any file system happens to
> > be frozen - not sure if that's a problem, but it's certainly different
> > from what we've been used to.
> 
> sync should not block on frozen fileystems. By definition, a frozen
> filesystem is a clean filesystem, and so sync should really just be
> skipping over them.

Just for record I agree with Dave. Sync on frozen fs should just return.
And freeze protection in iterate_supers() looks just wrong.

								Honza

> > +++ b/fs/super.c
> > @@ -514,10 +514,17 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
> >  		sb->s_count++;
> >  		spin_unlock(&sb_lock);
> >  
> > +		/*
> > +		 * Whatever we're going to do to the file system we have to
> > +		 * make sure that we'll not end up blocking on frozen file
> > +		 * system.
> > +		 */
> > +		sb_start_write(sb);
> >  		down_read(&sb->s_umount);
> >  		if (sb->s_root && (sb->s_flags & MS_BORN))
> >  			f(sb, arg);
> >  		up_read(&sb->s_umount);
> > +		sb_end_write(sb);
> >  
> >  		spin_lock(&sb_lock);
> >  		if (p)
> 
> That deadlocks sysrq-j (emergency thaw)...
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
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-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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