Re: [PATCH v5 2/2] buffer: record blockdev write errors in super_block that it backs

On Wed, 2020-04-15 at 16:06 +0200, Jan Kara wrote:

On Wed 15-04-20 08:13:00, Jeff Layton wrote:

quoted

From: Jeff Layton <redacted>

When syncing out a block device (a'la __sync_blockdev), any error
encountered will only be recorded in the bd_inode's mapping. When the
blockdev contains a filesystem however, we'd like to also record the
error in the super_block that's stored there.

Make mark_buffer_write_io_error also record the error in the
corresponding super_block when a writeback error occurs and the block
device contains a mounted superblock.

Since superblocks are RCU freed, hold the rcu_read_lock to ensure
that the superblock doesn't go away while we're marking it.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/buffer.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index f73276d746bb..2a4a5cc20418 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c

@@ -1154,12 +1154,19 @@ EXPORT_SYMBOL(mark_buffer_dirty);
 
 void mark_buffer_write_io_error(struct buffer_head *bh)
 {
+	struct super_block *sb;
+
 	set_buffer_write_io_error(bh);
 	/* FIXME: do we need to set this in both places? */
 	if (bh->b_page && bh->b_page->mapping)
 		mapping_set_error(bh->b_page->mapping, -EIO);
 	if (bh->b_assoc_map)
 		mapping_set_error(bh->b_assoc_map, -EIO);
+	rcu_read_lock();
+	sb = bh->b_bdev->bd_super;

You still need READ_ONCE() here. Otherwise the dereference below can still
result in refetch and NULL ptr deref.

								Honza

Huh? That seems like a really suspicious thing for the compiler/arch to
do. We are checking that sb isn't NULL before we dereference it. Doesn't
that imply a data dependency? How could the value of "sb" change after
that?

I'm also not sure I understand how using READ_ONCE really helps there if
we can't count on the value of a local variable not changing.

quoted

+	if (sb)
+		errseq_set(&sb->s_wb_err, -EIO);
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(mark_buffer_write_io_error);
 
-- 
2.25.2

-- 
Jeff Layton [off-list ref]