ext4: prevent parallel resizers by atomic bit ops

Before this patch, parallel resizers are allowed and protected by a
mutex lock, actually, there is no need to support parallel resizer, so
this patch prevents parallel resizers by atmoic bit ops, like
lock_page() and unlock_page() do.

To do this, the patch removed the mutex lock s_resize_lock from struct
ext4_sb_info and added a unsigned long field named s_resize_flags
which inidicates if there is a resizer.

Signed-off-by: Yongqiang Yang <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/ext4.h

···
 	struct journal_s *s_journal;
 	struct list_head s_orphan;
 	struct mutex s_orphan_lock;
-	struct mutex s_resize_lock;
+	unsigned long s_resize_flags;		/* Flags indicating if there
+						   is a resizer */
 	unsigned long s_commit_interval;
 	u32 s_max_batch_time;
 	u32 s_min_batch_time;
···
 					     EXT4_WQ_HASH_SZ])
 extern wait_queue_head_t ext4__ioend_wq[EXT4_WQ_HASH_SZ];
 extern struct mutex ext4__aio_mutex[EXT4_WQ_HASH_SZ];
+
+#define EXT4_RESIZING	0
+extern int ext4_resize_begin(struct super_block *sb);
+extern void ext4_resize_end(struct super_block *sb);
 
 #endif	/* __KERNEL__ */
 

fs/ext4/ioctl.c

···
 		struct super_block *sb = inode->i_sb;
 		int err, err2=0;
 
-		if (!capable(CAP_SYS_RESOURCE))
-			return -EPERM;
+		err = ext4_resize_begin(sb);
+		if (err)
+			return err;
 
 		if (get_user(n_blocks_count, (__u32 __user *)arg))
 			return -EFAULT;
···
 		if (err == 0)
 			err = err2;
 		mnt_drop_write(filp->f_path.mnt);
+		ext4_resize_end(sb);
 
 		return err;
 	}
···
 		struct super_block *sb = inode->i_sb;
 		int err, err2=0;
 
-		if (!capable(CAP_SYS_RESOURCE))
-			return -EPERM;
+		err = ext4_resize_begin(sb);
+		if (err)
+			return err;
 
 		if (copy_from_user(&input, (struct ext4_new_group_input __user *)arg,
 				sizeof(input)))
···
 		if (err == 0)
 			err = err2;
 		mnt_drop_write(filp->f_path.mnt);
+		ext4_resize_end(sb);
 
 		return err;
 	}

fs/ext4/resize.c

···
 
 #include "ext4_jbd2.h"
 
+int ext4_resize_begin(struct super_block *sb)
+{
+	int ret = 0;
+
+	if (!capable(CAP_SYS_RESOURCE))
+		return -EPERM;
+
+	if (test_and_set_bit_lock(EXT4_RESIZING, &EXT4_SB(sb)->s_resize_flags))
+		ret = -EBUSY;
+
+	return ret;
+}
+
+void ext4_resize_end(struct super_block *sb)
+{
+	clear_bit_unlock(EXT4_RESIZING, &EXT4_SB(sb)->s_resize_flags);
+	smp_mb__after_clear_bit();
+}
+
 #define outside(b, first, last)	((b) < (first) || (b) >= (last))
 #define inside(b, first, last)	((b) >= (first) && (b) < (last))
 
···
 	if (IS_ERR(handle))
 		return PTR_ERR(handle);
 
-	mutex_lock(&sbi->s_resize_lock);
-	if (input->group != sbi->s_groups_count) {
-		err = -EBUSY;
-		goto exit_journal;
-	}
+	BUG_ON(input->group != sbi->s_groups_count);
 
 	if (IS_ERR(bh = bclean(handle, sb, input->block_bitmap))) {
 		err = PTR_ERR(bh);
···
 	brelse(bh);
 
 exit_journal:
-	mutex_unlock(&sbi->s_resize_lock);
 	if ((err2 = ext4_journal_stop(handle)) && !err)
 		err = err2;
 
···
 		goto exit_put;
 	}
 
-	mutex_lock(&sbi->s_resize_lock);
-	if (input->group != sbi->s_groups_count) {
-		ext4_warning(sb, "multiple resizers run on filesystem!");
-		err = -EBUSY;
-		goto exit_journal;
-	}
-
 	if ((err = ext4_journal_get_write_access(handle, sbi->s_sbh)))
 		goto exit_journal;
 
···
         /*
          * OK, now we've set up the new group.  Time to make it active.
          *
-         * We do not lock all allocations via s_resize_lock
          * so we have to be safe wrt. concurrent accesses the group
          * data.  So we need to be careful to set all of the relevant
          * group descriptor data etc. *before* we enable the group.
···
 	 *
 	 * The precise rules we use are:
 	 *
-	 * * Writers of s_groups_count *must* hold s_resize_lock
-	 * AND
 	 * * Writers must perform a smp_wmb() after updating all dependent
 	 *   data and before modifying the groups count
 	 *
-	 * * Readers must hold s_resize_lock over the access
-	 * OR
 	 * * Readers must perform an smp_rmb() after reading the groups count
 	 *   and before reading any dependent data.
 	 *
···
 	ext4_handle_dirty_super(handle, sb);
 
 exit_journal:
-	mutex_unlock(&sbi->s_resize_lock);
 	if ((err2 = ext4_journal_stop(handle)) && !err)
 		err = err2;
 	if (!err) {
···
 	int err;
 	ext4_group_t group;
 
-	/* We don't need to worry about locking wrt other resizers just
-	 * yet: we're going to revalidate es->s_blocks_count after
-	 * taking the s_resize_lock below. */
 	o_blocks_count = ext4_blocks_count(es);
 
 	if (test_opt(sb, DEBUG))
···
 
 	if (n_blocks_count < o_blocks_count) {
 		ext4_warning(sb, "can't shrink FS - resize aborted");
-		return -EBUSY;
+		return -EINVAL;
 	}
 
 	/* Handle the remaining blocks in the last group only. */
···
 		goto exit_put;
 	}
 
-	mutex_lock(&EXT4_SB(sb)->s_resize_lock);
-	if (o_blocks_count != ext4_blocks_count(es)) {
-		ext4_warning(sb, "multiple resizers run on filesystem!");
-		mutex_unlock(&EXT4_SB(sb)->s_resize_lock);
-		ext4_journal_stop(handle);
-		err = -EBUSY;
-		goto exit_put;
-	}
-
 	if ((err = ext4_journal_get_write_access(handle,
 						 EXT4_SB(sb)->s_sbh))) {
 		ext4_warning(sb, "error %d on journal write access", err);
-		mutex_unlock(&EXT4_SB(sb)->s_resize_lock);
 		ext4_journal_stop(handle);
 		goto exit_put;
 	}
 	ext4_blocks_count_set(es, o_blocks_count + add);
-	mutex_unlock(&EXT4_SB(sb)->s_resize_lock);
 	ext4_debug("freeing blocks %llu through %llu\n", o_blocks_count,
 		   o_blocks_count + add);
 	/* We add the blocks to the bitmap and set the group need init bit */

fs/ext4/super.c

···
 
 	INIT_LIST_HEAD(&sbi->s_orphan); /* unlinked but open files */
 	mutex_init(&sbi->s_orphan_lock);
-	mutex_init(&sbi->s_resize_lock);
+	sbi->s_resize_flags = 0;
 
 	sb->s_root = NULL;