[PATCH] fsstack: fsstack_copy_inode_size locking



LTP's iogen01 doio tests used to hang nicely on 32-bit SMP when /tmp was a
unionfs mount of a tmpfs, i_size_read spinning forever, waiting for a lost
seqcount update: fixed by taking i_lock around i_size_write when 32-bit SMP.

But akpm was dissatisfied with the resulting patch: its lack of commentary,
the #ifs, the nesting around i_size_read, the lack of attention to i_blocks.
I promised to redo it with the general spin_lock_32bit() he proposed; but
disliked the result, partly because "32bit" obscures the real constraints,
which are best commented within fsstack_copy_inode_size itself.

This version adds those comments, and uses sizeof comparisons which the
compiler can optimize out, instead of CONFIG_SMP, CONFIG_LSF. BITS_PER_LONG.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---
Should follow mmotm's git-unionfs-fixup.patch

fs/stack.c | 58 +++++++++++++++++++++++++++++++------
include/linux/fs_stack.h | 3 -
2 files changed, 50 insertions(+), 11 deletions(-)

--- mmotm/fs/stack.c 2008-06-27 13:39:18.000000000 +0100
+++ linux/fs/stack.c 2008-06-27 18:24:19.000000000 +0100
@@ -19,16 +19,56 @@
* This function cannot be inlined since i_size_{read,write} is rather
* heavy-weight on 32-bit systems
*/
-void fsstack_copy_inode_size(struct inode *dst, const struct inode *src)
+void fsstack_copy_inode_size(struct inode *dst, struct inode *src)
{
-#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
- spin_lock(&dst->i_lock);
-#endif
- i_size_write(dst, i_size_read(src));
- dst->i_blocks = src->i_blocks;
-#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
- spin_unlock(&dst->i_lock);
-#endif
+ loff_t i_size;
+ blkcnt_t i_blocks;
+
+ /*
+ * i_size_read() includes its own seqlocking and protection from
+ * preemption (see include/linux/fs.h): we need nothing extra for
+ * that here, and prefer to avoid nesting locks than attempt to
+ * keep i_size and i_blocks in synch together.
+ */
+ i_size = i_size_read(src);
+
+ /*
+ * But if CONFIG_LSF (on 32-bit), we ought to make an effort to keep
+ * the two halves of i_blocks in synch despite SMP or PREEMPT - though
+ * stat's generic_fillattr() doesn't bother, and we won't be applying
+ * quotas (where i_blocks does become important) at the upper level.
+ *
+ * We don't actually know what locking is used at the lower level; but
+ * if it's a filesystem that supports quotas, it will be using i_lock
+ * as in inode_add_bytes(). tmpfs uses other locking, and its 32-bit
+ * is (just) able to exceed 2TB i_size with the aid of holes; but its
+ * i_blocks cannot carry into the upper long without almost 2TB swap -
+ * let's ignore that case.
+ */
+ if (sizeof(i_blocks) > sizeof(long))
+ spin_lock(&src->i_lock);
+ i_blocks = src->i_blocks;
+ if (sizeof(i_blocks) > sizeof(long))
+ spin_unlock(&src->i_lock);
+
+ /*
+ * If CONFIG_SMP on 32-bit, it's vital for fsstack_copy_inode_size()
+ * to hold some lock around i_size_write(), otherwise i_size_read()
+ * may spin forever (see include/linux/fs.h). We don't necessarily
+ * hold i_mutex when this is called, so take i_lock for that case.
+ *
+ * And if CONFIG_LSF (on 32-bit), continue our effort to keep the
+ * two halves of i_blocks in synch despite SMP or PREEMPT: use i_lock
+ * for that case too, and do both at once by combining the tests.
+ *
+ * There is none of this locking overhead in the 64-bit case.
+ */
+ if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long))
+ spin_lock(&dst->i_lock);
+ i_size_write(dst, i_size);
+ dst->i_blocks = i_blocks;
+ if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long))
+ spin_unlock(&dst->i_lock);
}
EXPORT_SYMBOL_GPL(fsstack_copy_inode_size);

--- mmotm/include/linux/fs_stack.h 2008-06-27 13:39:19.000000000 +0100
+++ linux/include/linux/fs_stack.h 2008-06-27 14:08:00.000000000 +0100
@@ -21,8 +21,7 @@

/* externs for fs/stack.c */
extern void fsstack_copy_attr_all(struct inode *dest, const struct inode *src);
-extern void fsstack_copy_inode_size(struct inode *dst,
- const struct inode *src);
+extern void fsstack_copy_inode_size(struct inode *dst, struct inode *src);

/* inlines */
static inline void fsstack_copy_attr_atime(struct inode *dest,
--
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