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Subject[PATCH 2/4] unionfs: 32-bit needs lock for i_size
LTP's iogen01 doio tests hang nicely on 32-bit SMP when /tmp is a unionfs
mount of a tmpfs. See the comment on i_size_write in linux/fs.h: it needs
to be locked, otherwise i_size_read can spin forever waiting for a lost
seqcount update.

Most filesystems are already holding i_mutex for this, but unionfs calls
fsstack_copy_inode_size from many places, not necessarily holding i_mutex.
Use the low-level i_lock within fsstack_copy_inode_size when 32-bit SMP.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
This works for me, to help get through LTP, but is not enough: what about
those places which use i_size_write in common code called by unionfs?
They ought to participate in this i_locking too. Needs an audit to
enumerate such places and decide what course to take.
Bigger question: is unionfs calling down to filesystems without holding
their i_mutex, where some may be relying on their i_mutex to be held?

fs/stack.c | 6 ++++++
1 file changed, 6 insertions(+)
--- 2.6.24-rc5-mm1/fs/stack.c 2007-12-05 10:38:38.000000000 +0000
+++ unionfs2/fs/stack.c 2007-12-05 16:50:15.000000000 +0000
@@ -21,8 +21,14 @@
*/
void fsstack_copy_inode_size(struct inode *dst, const struct inode *src)
{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+ spin_lock(&dst->i_lock);
+#endif
i_size_write(dst, i_size_read((struct inode *)src));
dst->i_blocks = src->i_blocks;
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+ spin_unlock(&dst->i_lock);
+#endif
}
EXPORT_SYMBOL_GPL(fsstack_copy_inode_size);


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