OGAWA Hirofumi <[email protected]> wrote:
>
> Hi,
>
> This race became a cause of oops, and can reproduce by the following.
>
> while true; do
> dd if=/dev/zero of=/dev/.static/dev/hdg1 bs=512 count=1000 & sync
> done
>
>
> This race condition was between __sync_single_inode() and iput().
>
> cpu0 (fs's inode) cpu1 (bdev's inode)
> ------------------------------------------------------------------------
> close("/dev/hda2")
> [...]
> __sync_single_inode()
> /* copy the bdev's ->i_mapping */
> mapping = inode->i_mapping;
>
> generic_forget_inode()
> bdev_clear_inode()
> /* restre the fs's ->i_mapping */
> inode->i_mapping = &inode->i_data;
> /* bdev's inode was freed */
> destroy_inode(inode);
>
> if (wait) {
> /* dereference a freed bdev's mapping->host */
> filemap_fdatawait(mapping); /* Oops */
>
> Since __sync_signle_inode() is only taking a ref-count of fs's inode,
> the another process can be close() and freeing the bdev's inode while
> writing fs's inode. So, __sync_signle_inode() accesses the freed
> ->i_mapping, oops.
>
> This patch takes ref-count of bdev's inode for fs's inode before
> setting a ->i_mapping, and the clear_inode() of fs's inode does iput().
> So, if fs's inode is still living, bdev's inode shouldn't be freed.
>
> This lifetime rule may be a poor, but very simple.
>
> Umm... should we use an another rule to free it more early?
> (e.g. if bdev's inode become I_FREEING, it should call bd_forget()
> before releasing the inode_lock. And some place should call
> igrab(->i_mapping->host->i_count) and iput())
>
>
> What do you think, comment?
Maybe. This code seems relatively straightforward though.
It would be preferable to have a couple of comments in there explaining
what the new refcounting is there for.
>
>...
>
> void bd_forget(struct inode *inode)
> {
> + struct block_device *old = NULL;
> +
> spin_lock(&bdev_lock);
> - if (inode->i_bdev)
> + if (inode->i_bdev) {
> + if (inode->i_sb != blockdev_superblock)
> + old = inode->i_bdev;
> __bd_forget(inode);
> + }
> spin_unlock(&bdev_lock);
> +
> + if (old)
> + iput(old->bd_inode);
> }
We're missing an atomic_inc(i_count) here?
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