linux-ext4 - [PATCH] ext4: fix freeze deadlock under IO

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Date:	Wed, 23 Jun 2010 14:37:27 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Phillip Susi <psusi@....rr.com>
CC:	Mike Snitzer <snitzer@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, Eric Sandeen <sandeen@...hat.com>,
	ext4 development <linux-ext4@...r.kernel.org>,
	Dave Chinner <dchinner@...hat.com>
Subject: [PATCH] ext4: fix freeze deadlock under IO

Commit 6b0310fbf087ad6 caused a regression resulting in deadlocks
when freezing a filesystem which had active IO; the vfs_check_frozen
level (SB_FREEZE_WRITE) did not let the freeze-related IO syncing
through.  Duh.

Changing the test to FREEZE_TRANS should let the normal freeze
syncing get through the fs, but still block any transactions from
starting once the fs is completely frozen.

I tested this by running fsstress in the background while periodically
snapshotting the fs and running fsck on the result.  I ran into
occasional deadlocks, but different ones.  I think this is a
fine fix for the problem at hand, and the other deadlocky things
will need more investigation.

Reported-by: Phillip Susi <psusi@....rr.com>
Signed-off-by: Eric Sandeen <sandeen@...hat.com>
---

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 4e8983a..a45ced9 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -241,7 +241,7 @@ handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)
 	if (sb->s_flags & MS_RDONLY)
 		return ERR_PTR(-EROFS);

-	vfs_check_frozen(sb, SB_FREEZE_WRITE);
+	vfs_check_frozen(sb, SB_FREEZE_TRANS);
 	/* Special case here: if the journal has aborted behind our
 	 * backs (eg. EIO in the commit thread), then we still need to
 	 * take the FS itself readonly cleanly. */
@@ -3491,7 +3491,7 @@ int ext4_force_commit(struct super_block *sb)

 	journal = EXT4_SB(sb)->s_journal;
 	if (journal) {
-		vfs_check_frozen(sb, SB_FREEZE_WRITE);
+		vfs_check_frozen(sb, SB_FREEZE_TRANS);
 		ret = ext4_journal_force_commit(journal);
 	}

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