Re: [PATCH v2] xfs: _{attr,data}_map_shared should take ILOCK_EXCL until iread_extents is completely done

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 11, 2023 at 11:49:34AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> While fuzzing the data fork extent count on a btree-format directory
> with xfs/375, I observed the following (excerpted) splat:
> 
> XFS: Assertion failed: xfs_isilocked(ip, XFS_ILOCK_EXCL), file: fs/xfs/libxfs/xfs_bmap.c, line: 1208
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 43192 at fs/xfs/xfs_message.c:104 assfail+0x46/0x4a [xfs]
> Call Trace:
>  <TASK>
>  xfs_iread_extents+0x1af/0x210 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_dir_walk+0xb8/0x190 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent_count_parent_dentries+0x41/0x80 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent_validate+0x199/0x2e0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent+0xdf/0x130 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_scrub_metadata+0x2b8/0x730 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_scrubv_metadata+0x38b/0x4d0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_ioc_scrubv_metadata+0x111/0x160 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_file_ioctl+0x367/0xf50 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  __x64_sys_ioctl+0x82/0xa0
>  do_syscall_64+0x2b/0x80
>  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> 
> The cause of this is a race condition in xfs_ilock_data_map_shared,
> which performs an unlocked access to the data fork to guess which lock
> mode it needs:
> 
> Thread 0                          Thread 1
> 
> xfs_need_iread_extents
> <observe no iext tree>
> xfs_ilock(..., ILOCK_EXCL)
> xfs_iread_extents
> <observe no iext tree>
> <check ILOCK_EXCL>
> <load bmbt extents into iext>
> <notice iext size doesn't
>  match nextents>
>                                   xfs_need_iread_extents
>                                   <observe iext tree>
>                                   xfs_ilock(..., ILOCK_SHARED)
> <tear down iext tree>
> xfs_iunlock(..., ILOCK_EXCL)
>                                   xfs_iread_extents
>                                   <observe no iext tree>
>                                   <check ILOCK_EXCL>
>                                   *BOOM*
> 
> Fix this race by adding a flag to the xfs_ifork structure to indicate
> that we have not yet read in the extent records and changing the
> predicate to look at the flag state, not if_height.  The memory barrier
> ensures that the flag will not be set until the very end of the
> function.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> v2: use smp_store_release per reviewer comments

Looks fine to me.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com