linux-kernel - Re: [PATCH 0/4] kill-the-bkl/reiserfs: fix some lock dependency inversions

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Date:	Wed, 16 Sep 2009 22:37:50 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Alexander Beregalov <a.beregalov@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Reiserfs <reiserfs-devel@...r.kernel.org>
Subject: Re: [PATCH 0/4] kill-the-bkl/reiserfs: fix some lock dependency
	inversions

On Tue, Sep 15, 2009 at 01:33:42AM +0400, Alexander Beregalov wrote:
> > Hi Alexander,
> >
> > It should be fixed now, still in the following tree:
> 
> Hi!
> Another one, similar:
> It is v2.6.31-3123-g99bc470 plus your 805031859(kill-the-bkl/reiserfs:
> panic in case of lock imbalance), UP.

Although I can't reproduce it, I think I see how that can happen.

On mount time, we have the following dependency:

reiserfs_lock -> bdev_mutex -> sysfs_mutex

which happens while calling journal_init_dev() because
we open the device there.

But also in case of mmap on a reiserfs filesystem we
may call reiserfs_readpages(), holding the reiserfs lock
while already holding mm->mmap_sem

The above dependency is then updated:

mmap_sem
     |
     |
     ------- reiserfs_lock -> bdev_mutex -> sysfs_mutex

And later, while doing a readdir() on a sysfs directory,
sysfs calls filldir, which might_fault, and then might grab
mmap_sem. filldir is called there while holding sys_mutex,
creating the new following dependency

sysfs_mutex -> mmap_sem

Hence the inversion.
It seems the deadlock can't ever happen, I even don't see
corner cases where it could happen.

But still, this dependency should disappear.

Could you please tell me if the following patch makes it shut down?

Otherwise, I may need your config. I don't know why, but I suspect
the lock_acquire(mmap_sem) in might_fault doesn't trigger needed the lockdep
check, although I have the appropriate debug config, at least it seems.
But anyway, it should also happen in my box but it doesn't...

Thanks!

The patch:

diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c
index d23d6d7..59f7a4c 100644
--- a/fs/reiserfs/journal.c
+++ b/fs/reiserfs/journal.c
@@ -2801,11 +2801,14 @@ int journal_init(struct super_block *sb, const char *j_dev_name,
 		goto free_and_return;
 	}

+	reiserfs_write_unlock(sb);
 	if (journal_init_dev(sb, journal, j_dev_name) != 0) {
 		reiserfs_warning(sb, "sh-462",
 				 "unable to initialize jornal device");
+		reiserfs_write_lock(sb);
 		goto free_and_return;
 	}
+	reiserfs_write_lock(sb);

 	rs = SB_DISK_SUPER_BLOCK(sb);

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