Re: linux-next: manual merge of the block tree with the ext4 tree

Sabrina Dubroca
2014-11-29T10:08:47Z

Hello,

[adding Jeremiah Mahler to CC]

2014-11-27, 14:53:47 +1100, Stephen Rothwell wrote:
> Hi Jens,
>
> Today's linux-next merge of the block tree got a conflict in
> fs/fs-writeback.c between commit ef7fdf5e8c87 ("vfs: add support for a
> lazytime mount option") from the ext4 tree and commit 9c6ac78eb352
> ("writeback: fix a subtle race condition in I_DIRTY clearing") from the
> block tree.
>
> I fixed it up (I took a guess, plese check - see below) and can carry
> the fix as necessary (no action is required).
>
> --
> Cheers,
> Stephen Rothwell sfr@canb.auug.org.au
>
> diff --cc fs/fs-writeback.c
> index 3d87174408ae,2d609a5fbfea..000000000000
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@@ -482,14 -479,30 +482,30 @@@ __writeback_single_inode(struct inode *
> * write_inode()
> */
> spin_lock(&inode->i_lock);
> - /* Clear I_DIRTY_PAGES if we've written out all dirty pages */
> - if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> - inode->i_state &= ~I_DIRTY_PAGES;
> +
> - dirty = inode->i_state & I_DIRTY;
> - inode->i_state &= ~I_DIRTY;
> + dirty = inode->i_state & I_DIRTY_INODE;
> + inode->i_state &= ~I_DIRTY_INODE;
> +
> + /*
> + * Paired with smp_mb() in __mark_inode_dirty(). This allows
> + * __mark_inode_dirty() to test i_state without grabbing i_lock -
> + * either they see the I_DIRTY bits cleared or we see the dirtied
> + * inode.
> + *
> + * I_DIRTY_PAGES is always cleared together above even if @mapping
> + * still has dirty pages. The flag is reinstated after smp_mb() if
> + * necessary. This guarantees that either __mark_inode_dirty()
> + * sees clear I_DIRTY_PAGES or we see PAGECACHE_TAG_DIRTY.
> + */
> + smp_mb();
> +
> + if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> + inode->i_state |= I_DIRTY_PAGES;
> +
> spin_unlock(&inode->i_lock);
> +
> /* Don't write the inode if only I_DIRTY_PAGES was set */
> - if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
> + if (dirty) {
> int err = write_inode(inode, wbc);
> if (ret == 0)
> ret = err;

I think there's a problem in your fix, Stephen.

I'm getting hangs at boot (strangely -- in QEMU -- only when booting
via grub, not when using -kernel) and during shutdown. Jeremiah seems
to have the same problem and his bisection led to the merge commit:
https://lkml.org/lkml/2014/11/29/17

The following solves both issues for me. I think it makes sense given
the #defines from ef7fdf5e8c87, since Tejun intended to clear
I_DIRTY_PAGES.

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index b70e45f45afa..6b2510d97a0a 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -484,7 +484,7 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
spin_lock(&inode->i_lock);

dirty = inode->i_state & I_DIRTY_INODE;
- inode->i_state &= ~I_DIRTY_INODE;
+ inode->i_state &= ~I_DIRTY;

/*
* Paired with smp_mb() in __mark_inode_dirty(). This allows

--
Thanks,
Sabrina