Re: [PATCH 4/4] fs: btrfs: fix a data race in btrfs_block_rsv_release()

[Jia-Ju Bai]

On 2020/5/13 6:18, David Sterba wrote:
> On Sat, May 09, 2020 at 01:34:31PM +0800, Jia-Ju Bai wrote:
> > The functions btrfs_block_rsv_release() and
> > btrfs_update_delayed_refs_rsv() are concurrently executed at runtime in
> > the following call contexts:
> >
> > Thread 1:
> >    btrfs_file_write_iter()
> >      btrfs_buffered_write()
> >        btrfs_delalloc_release_extents()
> >          btrfs_inode_rsv_release()
> >            __btrfs_block_rsv_release()
> >
> > Thread 2:
> >    finish_ordered_fn()
> >      btrfs_finish_ordered_io()
> >        insert_reserved_file_extent()
> >          __btrfs_drop_extents()
> >            btrfs_free_extent()
> >              btrfs_add_delayed_data_ref()
> >                btrfs_update_delayed_refs_rsv()
> >
> > In __btrfs_block_rsv_release():
> >    else if (... && !delayed_rsv->full)
> >
> > In btrfs_update_delayed_refs_rsv():
> >    spin_lock(&delayed_rsv->lock);
> >    delayed_rsv->size += num_bytes;
> >    delayed_rsv->full = 0;
> >    spin_unlock(&delayed_rsv->lock);
> >
> > Thus a data race for delayed_rsv->full can occur.
> > This race was found and actually reproduced by our conccurency fuzzer.
> >
> > To fix this race, the spinlock delayed_rsv->lock is used to
> > protect the access to delayed_rsv->full in btrfs_block_rsv_release().
> >
> > Signed-off-by: Jia-Ju Bai <baijiaju1990@xxxxxxxxx>
> > ---
> >   fs/btrfs/block-rsv.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> > index 27efec8f7c5b..89c53a7137b4 100644
> > --- a/fs/btrfs/block-rsv.c
> > +++ b/fs/btrfs/block-rsv.c
> > @@ -277,6 +277,11 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
> >   	struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
> >   	struct btrfs_block_rsv *delayed_rsv = &fs_info->delayed_refs_rsv;
> >   	struct btrfs_block_rsv *target = NULL;
> > +	unsigned short full = 0;
> > +
> > +	spin_lock(&delayed_rsv->lock);
> > +	full = delayed_rsv->full;
> > +	spin_unlock(&delayed_rsv->lock);
> >   
> >   	/*
> >   	 * If we are the delayed_rsv then push to the global rsv, otherwise dump
> > @@ -284,7 +289,7 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
> >   	 */
> >   	if (block_rsv == delayed_rsv)
> >   		target = global_rsv;
> > -	else if (block_rsv != global_rsv && !delayed_rsv->full)
> > +	else if (block_rsv != global_rsv && !full)
> This has been reported as suspicous
> https://lore.kernel.org/linux-btrfs/CAAwBoOJDjei5Hnem155N_cJwiEkVwJYvgN-tQrwWbZQGhFU=cA@xxxxxxxxxxxxxx/
>
> and there's an answer that this is racy but does not cause any
> unexpected behaviour.

Okay, thanks :)


Best wishes,
Jia-Ju Bai