Add a new rw_semaphore to protect bmap against truncate.  Previous
i_alloc_sem was abused for this, but it's going away in this series.

Note that we can't simply use i_mutex, given that the swapon code
calls ->bmap under it.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/fs/fat/inode.c
===================================================================
--- linux-2.6.orig/fs/fat/inode.c	2011-06-20 21:28:19.707963855 +0200
+++ linux-2.6/fs/fat/inode.c	2011-06-20 21:29:25.031293882 +0200
@@ -224,9 +224,9 @@ static sector_t _fat_bmap(struct address
 	sector_t blocknr;
 
 	/* fat_get_cluster() assumes the requested blocknr isn't truncated. */
-	down_read(&mapping->host->i_alloc_sem);
+	down_read(&MSDOS_I(mapping->host)->truncate_lock);
 	blocknr = generic_block_bmap(mapping, block, fat_get_block);
-	up_read(&mapping->host->i_alloc_sem);
+	up_read(&MSDOS_I(mapping->host)->truncate_lock);
 
 	return blocknr;
 }
@@ -510,6 +510,8 @@ static struct inode *fat_alloc_inode(str
 	ei = kmem_cache_alloc(fat_inode_cachep, GFP_NOFS);
 	if (!ei)
 		return NULL;
+
+	init_rwsem(&ei->truncate_lock);
 	return &ei->vfs_inode;
 }
 
Index: linux-2.6/fs/fat/fat.h
===================================================================
--- linux-2.6.orig/fs/fat/fat.h	2011-06-20 21:28:19.724630522 +0200
+++ linux-2.6/fs/fat/fat.h	2011-06-20 21:29:25.034627215 +0200
@@ -109,6 +109,7 @@ struct msdos_inode_info {
 	int i_attrs;		/* unused attribute bits */
 	loff_t i_pos;		/* on-disk position of directory entry or 0 */
 	struct hlist_node i_fat_hash;	/* hash by i_location */
+	struct rw_semaphore truncate_lock; /* protect bmap against truncate */
 	struct inode vfs_inode;
 };
 
Index: linux-2.6/fs/fat/file.c
===================================================================
--- linux-2.6.orig/fs/fat/file.c	2011-06-20 21:28:19.744630521 +0200
+++ linux-2.6/fs/fat/file.c	2011-06-20 21:29:54.501292390 +0200
@@ -429,8 +429,10 @@ int fat_setattr(struct dentry *dentry, s
 	}
 
 	if (attr->ia_valid & ATTR_SIZE) {
+		down_write(&MSDOS_I(inode)->truncate_lock);
 		truncate_setsize(inode, attr->ia_size);
 		fat_truncate_blocks(inode, attr->ia_size);
+		up_write(&MSDOS_I(inode)->truncate_lock);
 	}
 
 	setattr_copy(inode, attr);

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