linux-kernel - Re: [RFC v8][PATCH 0/12] Kernel based checkpoint/restart

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Date:	Tue, 4 Nov 2008 15:38:05 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Linus Torvalds <torvalds@...l.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC v8][PATCH 0/12] Kernel based checkpoint/restart

Quoting Oren Laadan (orenl@...columbia.edu):
> Basic checkpoint-restart [C/R]: v8 adds support for "external" checkpoint
> and improves documentation. Older announcements below.

Finally!

>From 8edab186b605f7dddd612e581204f1ad8fd766be Mon Sep 17 00:00:00 2001
From: Serge Hallyn <serue@...ibm.com>
Date: Tue, 4 Nov 2008 15:28:01 -0600
Subject: [PATCH 1/1] cr: fix use of __d_path()

__d_path():
	1. should be used under dcache_lock
	2. can change root->{mnt,dentry} without changing refcounts
The second point was the cause of my BUGs.  The ctx->root was passed
in, and do_checkpoint() had taken a path_get on the vfsroot.  So now
at cleanup it was doing path_put() using another mnt+dentry.

(Why they are different, I'm not sure - but my guess would be that
stdin or stdout is inherited from the parent task in parent mntns,
hence file->mnt is different from root->mnt as it's a different
namespace.)

Signed-off-by: Serge Hallyn <serue@...ibm.com>
---
 checkpoint/checkpoint.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/checkpoint/checkpoint.c b/checkpoint/checkpoint.c
index 173b637..7f0c1e7 100644
--- a/checkpoint/checkpoint.c
+++ b/checkpoint/checkpoint.c
@@ -70,9 +70,20 @@ static char *
 cr_fill_fname(struct path *path, struct path *root, char *buf, int *n)
 {
 	char *fname;
+	struct path root2;
+
+	root2.mnt = root->mnt;
+	root2.dentry = root->dentry;
 
 	BUG_ON(!buf);
-	fname = __d_path(path, root, buf, *n);
+	spin_lock(&dcache_lock);
+	fname = __d_path(path, &root2, buf, *n);
+	spin_unlock(&dcache_lock);
+	if (root2.mnt != root->mnt)
+		printk(KERN_NOTICE "%s: mnt changed\n", __func__);
+	if (root2.dentry != root->dentry)
+		printk(KERN_NOTICE "%s: dentry changed\n", __func__);
+	fname = buf+10;
 	if (!IS_ERR(fname))
 		*n = (buf + (*n) - fname);
 	return fname;
-- 
1.5.6.3

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