Re: spurious -ENOSPC on XFS

Mikulas Patocka
2009-01-29T16:39:56Z

On Sat, 24 Jan 2009, Dave Chinner wrote:

> On Fri, Jan 23, 2009 at 03:14:30PM -0500, Mikulas Patocka wrote:
> >
> >
> > On Thu, 22 Jan 2009, Christoph Hellwig wrote:
> >
> > > On Thu, Jan 22, 2009 at 03:59:13PM -0500, Christoph Hellwig wrote:
> > > > On Wed, Jan 21, 2009 at 10:24:22AM +1100, Dave Chinner wrote:
> > > > > Right, so you need to use internal xfs sync functions that don't
> > > > > have these problems. That is:
> > > > >
> > > > > error = xfs_sync_inodes(ip->i_mount, SYNC_DELWRI|SYNC_WAIT);
> > > > >
> > > > > will do a blocking flush of all the inodes without deadlocks occurring.
> > > > > Then you can remove the 500ms wait.
> > > >
> > > > I've given this a try with Eric's testcase from #724 in the oss bugzilla,
> > > > but it's not enough yet. I thinks that's because SYNC_WAIT is rather
> > > > meaningless for data writeout, and we need SYNC_IOWAIT instead. The
> > > > patch below gets the testcase working for me:
> > >
> > > Actually I still see failures happing sometimes. I guess tha'ts because
> > > our flush is still asynchronous due to the schedule_work..
> >
> > If I placed
> > xfs_sync_inodes(ip->i_mount, SYNC_DELWRI);
> > xfs_sync_inodes(ip->i_mount, SYNC_DELWRI | SYNC_IOWAIT);
> > directly to xfs_flush_device, I got lock dependency warning (though not a
> > real deadlock).
>
> Same reason memory reclaim gives lockdep warnings on XFS - we're
> recursing into operations that take inode locks while we currently
> hold an inode lock. However, it shouldn't deadlock because
> we should ever try to take the iolock on the inode that we current
> hold it on.
>
> > 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@fromorbit.com

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