VFS/fsstack: handle 32-bit smp + preempt + large files in fsstack_copy_inode_size

Copy the inode size and blocks from one inode to another correctly on 32-bit
systems with CONFIG_SMP, CONFIG_PREEMPT, or CONFIG_LBDAF.  Use proper inode
spinlocks only when i_size/i_blocks cannot fit in one 32-bit word.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 52 insertions, 4 deletions

diff --git a/fs/stack.c b/fs/stack.c
index 0e20e43ad740..4a6f7f440658 100644
--- a/fs/stack.c
+++ b/fs/stack.c
@@ -7,10 +7,58 @@
  * 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)
 {
-        i_size_write(dst, i_size_read((struct inode *)src));
-        dst->i_blocks = src->i_blocks;
+        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 sync together.
+         */
+        i_size = i_size_read(src);
+
+        /*
+         * But if CONFIG_LBDAF (on 32-bit), we ought to make an effort to
+         * keep the two halves of i_blocks in sync 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 or CONFIG_PREEMPT 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_LBADF (on 32-bit), continue our effort to keep the
+         * two halves of i_blocks in sync 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);
 
diff --git a/include/linux/fs_stack.h b/include/linux/fs_stack.h
index aa60311900dd..da317c7163ab 100644
--- a/include/linux/fs_stack.h
+++ b/include/linux/fs_stack.h
@@ -9,7 +9,7 @@
 
 /* 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,