Re: [PATCH block/for-linus] writeback: fix a subtle race condition in I_DIRTY clearing

[Tejun Heo]

On Fri, Oct 24, 2014 at 03:38:21PM -0400, Tejun Heo wrote:
> After invoking ->dirty_inode(), __mark_inode_dirty() does smp_mb() and
> tests inode->i_state locklessly to see whether it already has all the
> necessary I_DIRTY bits set.  The comment above the barrier doesn't
> contain any useful information - memory barriers can't ensure "changes
> are seen by all cpus" by itself.
> 
> And it sure enough was broken.  Please consider the following
> scenario.
> 
>  CPU 0					CPU 1
>  -------------------------------------------------------------------------------
> 
> 					enters __writeback_single_inode()
> 					grabs inode->i_lock
> 					tests PAGECACHE_TAG_DIRTY which is clear
>  enters __set_page_dirty()
>  grabs mapping->tree_lock
>  sets PAGECACHE_TAG_DIRTY
>  releases mapping->tree_lock
>  leaves __set_page_dirty()
> 
>  enters __mark_inode_dirty()
>  smp_mb()
>  sees I_DIRTY_PAGES set
>  leaves __mark_inode_dirty()
> 					clears I_DIRTY_PAGES
> 					releases inode->i_lock
> 
> Now @inode has dirty pages w/ I_DIRTY_PAGES clear.  This doesn't seem
> to lead to an immediately critical problem because requeue_inode()
> later checks PAGECACHE_TAG_DIRTY instead of I_DIRTY_PAGES when
> deciding whether the inode needs to be requeued for IO and there are
> enough unintentional memory barriers inbetween, so while the inode
> ends up with inconsistent I_DIRTY_PAGES flag, it doesn't fall off the
> IO list.
> 
> The lack of explicit barrier may also theoretically affect the other
> I_DIRTY bits which deal with metadata dirtiness.  There is no
> guarantee that a strong enough barrier exists between
> I_DIRTY_[DATA]SYNC clearing and write_inode() writing out the dirtied
> inode.  Filesystem inode writeout path likely has enough stuff which
> can behave as full barrier but it's theoretically possible that the
> writeout may not see all the updates from ->dirty_inode().
> 
> Fix it by adding an explicit smp_mb() after I_DIRTY clearing.  Note
> that I_DIRTY_PAGES needs a special treatment as it always needs to be
> cleared to be interlocked with the lockless test on
> __mark_inode_dirty() side.  It's cleared unconditionally and
> reinstated after smp_mb() if the mapping still has dirty pages.
> 
> Also add comments explaining how and why the barriers are paired.
> 
> Lightly tested.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Jens, can you please route this one?

Thanks.

-- 
tejun
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