xfs: shrink failure needs to hold AGI buffer
Chandan reported a AGI/AGF lock order hang on xfs/168 during recent testing. The cause of the problem was the task running xfs_growfs to shrink the filesystem. A failure occurred trying to remove the free space from the btrees that the shrink would make disappear, and that meant it ran the error handling for a partial failure. This error path involves restoring the per-ag block reservations, and that requires calculating the amount of space needed to be reserved for the free inode btree. The growfs operation hung here: [18679.536829] down+0x71/0xa0 [18679.537657] xfs_buf_lock+0xa4/0x290 [xfs] [18679.538731] xfs_buf_find_lock+0xf7/0x4d0 [xfs] [18679.539920] xfs_buf_lookup.constprop.0+0x289/0x500 [xfs] [18679.542628] xfs_buf_get_map+0x2b3/0xe40 [xfs] [18679.547076] xfs_buf_read_map+0xbb/0x900 [xfs] [18679.562616] xfs_trans_read_buf_map+0x449/0xb10 [xfs] [18679.569778] xfs_read_agi+0x1cd/0x500 [xfs] [18679.573126] xfs_ialloc_read_agi+0xc2/0x5b0 [xfs] [18679.578708] xfs_finobt_calc_reserves+0xe7/0x4d0 [xfs] [18679.582480] xfs_ag_resv_init+0x2c5/0x490 [xfs] [18679.586023] xfs_ag_shrink_space+0x736/0xd30 [xfs] [18679.590730] xfs_growfs_data_private.isra.0+0x55e/0x990 [xfs] [18679.599764] xfs_growfs_data+0x2f1/0x410 [xfs] [18679.602212] xfs_file_ioctl+0xd1e/0x1370 [xfs] trying to get the AGI lock. The AGI lock was held by a fstress task trying to do an inode allocation, and it was waiting on the AGF lock to allocate a new inode chunk on disk. Hence deadlock. The fix for this is for the growfs code to hold the AGI over the transaction roll it does in the error path. It already holds the AGF locked across this, and that is what causes the lock order inversion in the xfs_ag_resv_init() call. Reported-by:Chandan Babu R <chandanbabu@kernel.org> Fixes: 46141dc8 ("xfs: introduce xfs_ag_shrink_space()") Signed-off-by: