Re: spurious -ENOSPC on XFS

[Mikulas Patocka]

> > > So synchronous flushing definitely needs some thinking and lock 
> > > rearchitecting.
> > 
> > No, not at all. At most the grabbing of the iolock in
> > xfs_sync_inodes_ag() needs to become a trylock....
> > 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@xxxxxxxxxxxxx
> 
> You are wrong, the comments in the code are right. It really deadlocks if 
> it is modified to use synchronous wait for the end of the flush and if 
> the flush is done with xfs_sync_inodes:
> 
> xfssyncd      D 0000000000606808     0  4819      2
> Call Trace:
>  [0000000000606788] rwsem_down_failed_common+0x1ac/0x1d8
>  [0000000000606808] rwsem_down_read_failed+0x24/0x34
>  [0000000000606848] __down_read+0x30/0x40
>  [0000000000468a64] down_read_nested+0x48/0x58
>  [00000000100e6cc8] xfs_ilock+0x48/0x8c [xfs]
>  [000000001011018c] xfs_sync_inodes_ag+0x17c/0x2dc [xfs]
>  [000000001011034c] xfs_sync_inodes+0x60/0xc4 [xfs]
>  [00000000101103c4] xfs_flush_device_work+0x14/0x2c [xfs]
>  [000000001010ff1c] xfssyncd+0x110/0x174 [xfs]
>  [000000000046556c] kthread+0x54/0x88
>  [000000000042b2a0] kernel_thread+0x3c/0x54
>  [00000000004653b8] kthreadd+0xac/0x160
> dd            D 00000000006045c4     0  4829   2500
> Call Trace:
>  [00000000006047d8] schedule_timeout+0x24/0xb8
>  [00000000006045c4] wait_for_common+0xe4/0x14c
>  [0000000000604700] wait_for_completion+0x1c/0x2c
>  [00000000101106b0] xfs_flush_device+0x68/0x88 [xfs]
>  [00000000100edba4] xfs_flush_space+0xa8/0xd0 [xfs]
>  [00000000100edec0] xfs_iomap_write_delay+0x1bc/0x228 [xfs]
>  [00000000100ee4b8] xfs_iomap+0x1c4/0x2e0 [xfs]
>  [0000000010104f04] __xfs_get_blocks+0x74/0x240 [xfs]
>  [000000001010512c] xfs_get_blocks+0x24/0x38 [xfs]
>  [00000000004d05f0] __block_prepare_write+0x184/0x41c
>  [00000000004d095c] block_write_begin+0x84/0xe8
>  [000000001010447c] xfs_vm_write_begin+0x3c/0x50 [xfs]
>  [0000000000485258] generic_file_buffered_write+0xe8/0x28c
>  [000000001010cec4] xfs_write+0x40c/0x604 [xfs]
>  [0000000010108d3c] xfs_file_aio_write+0x74/0x84 [xfs]
>  [00000000004ae70c] do_sync_write+0x8c/0xdc
> 
> Mikulas

BTW. I modified it to use completion to wait for the end of the sync work 
(instead of that fixed 500ms wait) and got this deadlock quite quickly.

filemap_flush used to deadlock, I disabled it and used only 
xfs_sync_inodes, but xfs_sync_inodes also deadlocks.

Mikulas
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