tmpfs: no need to use i_lock

2.6.36's 7e496299 ("tmpfs: make tmpfs scalable with percpu_counter for
used blocks") to make tmpfs scalable with percpu_counter used
inode->i_lock in place of sbinfo->stat_lock around i_blocks updates; but
that was adverse to scalability, and unnecessary, since info->lock is
already held there in the fast paths.

Remove those uses of i_lock, and add info->lock in the three error paths
where it's then needed across shmem_free_blocks().  It's not actually
needed across shmem_unacct_blocks(), but they're so often paired that it
looks wrong to split them apart.
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -241,9 +241,7 @@ static void shmem_free_blocks(struct inode *inode, long pages)
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	if (sbinfo->max_blocks) {
 		percpu_counter_add(&sbinfo->used_blocks, -pages);
-		spin_lock(&inode->i_lock);
 		inode->i_blocks -= pages*BLOCKS_PER_PAGE;
-		spin_unlock(&inode->i_lock);
 	}
 }
 
@@ -432,9 +430,7 @@ static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long
 						sbinfo->max_blocks - 1) >= 0)
 				return ERR_PTR(-ENOSPC);
 			percpu_counter_inc(&sbinfo->used_blocks);
-			spin_lock(&inode->i_lock);
 			inode->i_blocks += BLOCKS_PER_PAGE;
-			spin_unlock(&inode->i_lock);
 		}
 
 		spin_unlock(&info->lock);
@@ -1421,9 +1417,7 @@ static int shmem_getpage(struct inode *inode, unsigned long idx,
 			    shmem_acct_block(info->flags))
 				goto nospace;
 			percpu_counter_inc(&sbinfo->used_blocks);
-			spin_lock(&inode->i_lock);
 			inode->i_blocks += BLOCKS_PER_PAGE;
-			spin_unlock(&inode->i_lock);
 		} else if (shmem_acct_block(info->flags))
 			goto nospace;
 
@@ -1434,8 +1428,10 @@ static int shmem_getpage(struct inode *inode, unsigned long idx,
 				spin_unlock(&info->lock);
 				filepage = shmem_alloc_page(gfp, info, idx);
 				if (!filepage) {
+					spin_lock(&info->lock);
 					shmem_unacct_blocks(info->flags, 1);
 					shmem_free_blocks(inode, 1);
+					spin_unlock(&info->lock);
 					error = -ENOMEM;
 					goto failed;
 				}
@@ -1449,8 +1445,10 @@ static int shmem_getpage(struct inode *inode, unsigned long idx,
 					current->mm, GFP_KERNEL);
 				if (error) {
 					page_cache_release(filepage);
+					spin_lock(&info->lock);
 					shmem_unacct_blocks(info->flags, 1);
 					shmem_free_blocks(inode, 1);
+					spin_unlock(&info->lock);
 					filepage = NULL;
 					goto failed;
 				}
@@ -1480,10 +1478,10 @@ static int shmem_getpage(struct inode *inode, unsigned long idx,
 			 * be done automatically.
 			 */
 			if (ret) {
-				spin_unlock(&info->lock);
-				page_cache_release(filepage);
 				shmem_unacct_blocks(info->flags, 1);
 				shmem_free_blocks(inode, 1);
+				spin_unlock(&info->lock);
+				page_cache_release(filepage);
 				filepage = NULL;
 				if (error)
 					goto failed;