linux-kernel - Re: XFS assertion from truncate. (3.10-rc2)

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Message-ID: <20130522161946.GA25906@redhat.com>
Date:	Wed, 22 May 2013 12:19:46 -0400
From:	Dave Jones <davej@...hat.com>
To:	Dave Chinner <david@...morbit.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: XFS assertion from truncate. (3.10-rc2)

On Wed, May 22, 2013 at 10:22:52AM -0400, Dave Jones wrote:
 > On Wed, May 22, 2013 at 03:51:47PM +1000, Dave Chinner wrote:
 > 
 >  > > Tomorrow I'll also try running some older kernels with the same
 >  > > options to see if it's something new, or an older bug. This is a
 >  > > new machine, so it may be something that's been around for a
 >  > > while, and for whatever reason, my other machines don't hit
 >  > > this.
 >  > 
 >  > Another thing that just occurred to me - what compiler are you
 >  > using?  We had a report last week on #xfs that xfsdump was failing
 >  > with bad checksums because of link time optimisation (LTO) in
 >  > gcc-4.8.0. When they turned that off, everything worked fine. So if
 >  > you are using 4.8.0, perhaps trying a different compiler might be a
 >  > good idea, too.
 > 
 > Yeah, this is 4.8.0. This box is running F19-beta. 
 > I managed to shoehorn the gcc-4.7 from f18 on there though.
 > Bug reproduced instantly, so I think we can rule out compiler.
 > 
 > I ran 3.9 with the same debug options. Seems stable.
 > I'll do a bisect.

good news.  It wasn't until I started bisecting I realised I was still
carrying this patch from you to fix slab corruption I was seeing.

It seems to be the culprit (or is masking another problem -- I had to apply
it at each step of the bisect to get past the slab corruption bug).

	Dave

--- /home/davej/src/kernel/git-trees/linux/fs/xfs/xfs_extfree_item.c	2013-05-03 10:03:05.331370231 -0400
+++ linux-dj/fs/xfs/xfs_extfree_item.c	2013-05-07 20:46:42.389262296 -0400
@@ -305,10 +305,22 @@ xfs_efi_release(xfs_efi_log_item_t	*efip
 {
 	ASSERT(atomic_read(&efip->efi_next_extent) >= nextents);
 	if (atomic_sub_and_test(nextents, &efip->efi_next_extent)) {
+		int recovered;
+
+		/*
+		 * __xfs_efi_release() can release the last reference to the EFI
+		 * and free it, so it is unsafe to reference it after we've
+		 * released the reference. The only case this is safe to do is
+		 * if we are in recovery and the XFS_EFI_RECOVERED bit is set,
+		 * meaning that we have two references to release. Check the
+		 * recovered bit before the initial release, as we cannot
+		 * reliably check it afterwards.
+		 */
+		recovered = test_bit(XFS_EFI_RECOVERED, &efip->efi_flags);
 		__xfs_efi_release(efip);
 
 		/* recovery needs us to drop the EFI reference, too */
-		if (test_bit(XFS_EFI_RECOVERED, &efip->efi_flags))
+		if (recovered)
 			__xfs_efi_release(efip);
 	}
 }
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