[PATCH 07/13] xfs: document the invalidate_bdev call in invalidate_bdev

[Christoph Hellwig <hch@lst.de>]

Copy and paste the commit message from Darrick into a comment to explain
the seemly odd invalidate_bdev in xfs_shutdown_devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_super.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 4ae3b01ed038c7..c169beb0d8cab3 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -399,6 +399,32 @@ STATIC void
 xfs_shutdown_devices(
 	struct xfs_mount	*mp)
 {
+	/*
+	 * Udev is triggered whenever anyone closes a block device or unmounts
+	 * a file systemm on a block device.
+	 * The default udev rules invoke blkid to read the fs super and create
+	 * symlinks to the bdev under /dev/disk.  For this, it uses buffered
+	 * reads through the page cache.
+	 *
+	 * xfs_db also uses buffered reads to examine metadata.  There is no
+	 * coordination between xfs_db and udev, which means that they can run
+	 * concurrently.  Note there is no coordination between the kernel and
+	 * blkid either.
+	 *
+	 * On a system with 64k pages, the page cache can cache the superblock
+	 * and the root inode (and hence the root directory) with the same 64k
+	 * page.  If udev spawns blkid after the mkfs and the system is busy
+	 * enough that it is still running when xfs_db starts up, they'll both
+	 * read from the same page in the pagecache.
+	 *
+	 * The unmount writes updated inode metadata to disk directly.  The XFS
+	 * buffer cache does not use the bdev pagecache, nor does it invalidate
+	 * the pagecache on umount.  If the above scenario occurs, the pagecache
+	 * no longer reflects what's on disk, xfs_db reads the stale metadata,
+	 * and fails to find /a.  Most of the time this succeeds because closing
+	 * a bdev invalidates the page cache, but when processes race, everyone
+	 * loses.
+	 */
 	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp) {
 		blkdev_issue_flush(mp->m_logdev_targp->bt_bdev);
 		invalidate_bdev(mp->m_logdev_targp->bt_bdev);
-- 
2.39.2