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Date: Thu, 13 Jan 2011 11:03:46 +0800
From: Ian Kent <raven@...maw.net>
To: aelder@....com
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Nick Piggin <npiggin@...nel.dk>,
Al Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [announce] vfs-scale git tree update
On Thu, 2011-01-13 at 10:23 +0800, Ian Kent wrote:
> On Wed, 2011-01-12 at 14:11 -0600, Alex Elder wrote:
> > On Wed, 2011-01-12 at 12:15 +0800, Ian Kent wrote:
> > > On Wed, 2011-01-12 at 11:59 +0800, Ian Kent wrote:
> > > > On Tue, 2011-01-11 at 11:57 -0600, Alex Elder wrote:
> > > > > On Tue, 2011-01-11 at 08:51 -0800, Linus Torvalds wrote:
> > > > > > On Tue, Jan 11, 2011 at 8:34 AM, Alex Elder <aelder@....com> wrote:
> > > > > > >
> > > > > > > FYI, when using this code, as merged by Linus, I hit the
> > > > > > > BUG_ON() at the beginning of d_set_d_op() when it's called
> > > > > > > by autofs4_dir_mkdir(). I managed to work around it by
> > > > > > > just commenting out those BUG_ON() calls but it's something
> > > > > > > that ought to get addressed properly.
> > > > > >
> > > > > > Yeah, removing the BUG_ON() isn't the right thing to do - it means
> > > > > > that autofs4 is obviously setting the dentry ops twice for the same
> > > > > > dentry.
> > > > > >
> > > > > > Possibly the thing could be relaxed to allow setting the _same_ d_op
> > > > > > pointer, ie do something like
> > > > > >
> > > > > > if (dentry->d_op == op)
> > > > > > return;
> > > > > >
> > > > > > at the top of that function. But looking at it, I don't think that
> > > > > > fixes the autofs4 issue.
> > > > >
> > > > > That's easy enough, but it seems everybody else ensures
> > > > > this gets done just once per dentry, and it would be nice
> > > > > to preserve that "tightness" if possible.
> > > > >
> > > > > > The fact that autofs4 does "d_add()" before it sets the d_ops (or
> > > > > > other dentry state, for that matter) looks a bit scary. To me that
> > > > > > smells like it might get a dentry lookup hit before it's actually
> > > > > > fully done.
> > > > >
> > > > > Agreed.
> > > >
> > > > Isn't the parent i_mutex held during mkdir()?
> > > > Still the order can be changed, of course.
> > > >
> > > > >
> > > > > > Does it make any difference if you move the various d_add() calls down
> > > > > > to the end of the functions to when the "dentry" has really been
> > > > > > instantiated?
> > > > >
> > > > > Looking at it quickly, I don't think that would matter for
> > > > > the case at hand. I.e., that might be safer but it doesn't
> > > > > address the fact that these fields are getting initialized
> > > > > multiple times.
> > > >
> > > > Yeah, a hangover from changes done over time.
> > > > Not setting the dentry op in ->lookup() should fix this.
> > >
> > > Could you try this patch please.
> >
> > OK, sorry for the delay. I tried the patch. I applied
> > it against 4162cf64973df51fc885825bc9ca4d055891c49f,
> > which is the linus/master branch I had on hand. This
> > time I got a different failure due to a null pointer
> > dereference. Console capture below. I can log
> > in still but the boot sequence never got to the
> > login prompt on the console as it normally does.
>
>
> Sorry, that is rather an obvious mistake on my part.
> There's a call to d_op->d_revalidate() just below where the dentry
> operations are set. I'm tempted to just call the revalidate function
> directly since it is always called anyway. But let me check we've done
> in the our current of tree autofs patch series first.
Ha, there is no revalidate in the current not yet merged autofs.
So I think we just need to work around it.
Try this and see if it resolves the issue.
autofs4 - set dentry op in ->lookup() only
From: Ian Kent <raven@...maw.net>
With the introduction of the vfs-scale patch series setting dentry
operations more than once (or changing them) triggers a BUG_ON().
Since the two dentry operations used are the same, just set the
operations in ->lookup() and remove the set in ->symlink() and
->mkdir().
Also, move the d_add() in ->symlink() and ->mkdir() to the end of
the function.
---
fs/autofs4/root.c | 40 +++++++---------------------------------
1 files changed, 7 insertions(+), 33 deletions(-)
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 651e4ef..0897706 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -419,12 +419,6 @@ void autofs4_dentry_release(struct dentry *de)
}
}
-/* For dentries of directories in the root dir */
-static const struct dentry_operations autofs4_root_dentry_operations = {
- .d_revalidate = autofs4_revalidate,
- .d_release = autofs4_dentry_release,
-};
-
/* For other dentries */
static const struct dentry_operations autofs4_dentry_operations = {
.d_revalidate = autofs4_revalidate,
@@ -568,19 +562,6 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
ino = autofs4_dentry_ino(dentry);
} else {
/*
- * Mark the dentry incomplete but don't hash it. We do this
- * to serialize our inode creation operations (symlink and
- * mkdir) which prevents deadlock during the callback to
- * the daemon. Subsequent user space lookups for the same
- * dentry are placed on the wait queue while the daemon
- * itself is allowed passage unresticted so the create
- * operation itself can then hash the dentry. Finally,
- * we check for the hashed dentry and return the newly
- * hashed dentry.
- */
- d_set_d_op(dentry, &autofs4_root_dentry_operations);
-
- /*
* And we need to ensure that the same dentry is used for
* all following lookup calls until it is hashed so that
* the dentry flags are persistent throughout the request.
@@ -589,6 +570,8 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
if (!ino)
return ERR_PTR(-ENOMEM);
+ d_set_d_op(dentry, &autofs4_dentry_operations);
+
dentry->d_fsdata = ino;
ino->dentry = dentry;
@@ -714,12 +697,6 @@ static int autofs4_dir_symlink(struct inode *dir,
kfree(ino);
return -ENOMEM;
}
- d_add(dentry, inode);
-
- if (dir == dir->i_sb->s_root->d_inode)
- d_set_d_op(dentry, &autofs4_root_dentry_operations);
- else
- d_set_d_op(dentry, &autofs4_dentry_operations);
dentry->d_fsdata = ino;
ino->dentry = dget(dentry);
@@ -732,6 +709,8 @@ static int autofs4_dir_symlink(struct inode *dir,
ino->u.symlink = cp;
dir->i_mtime = CURRENT_TIME;
+ d_add(dentry, inode);
+
return 0;
}
@@ -849,12 +828,6 @@ static int autofs4_dir_mkdir(struct inode *dir, struct dentry *dentry, int mode)
kfree(ino);
return -ENOMEM;
}
- d_add(dentry, inode);
-
- if (dir == dir->i_sb->s_root->d_inode)
- d_set_d_op(dentry, &autofs4_root_dentry_operations);
- else
- d_set_d_op(dentry, &autofs4_dentry_operations);
dentry->d_fsdata = ino;
ino->dentry = dget(dentry);
@@ -866,6 +839,8 @@ static int autofs4_dir_mkdir(struct inode *dir, struct dentry *dentry, int mode)
inc_nlink(dir);
dir->i_mtime = CURRENT_TIME;
+ d_add(dentry, inode);
+
return 0;
}
@@ -944,8 +919,7 @@ static inline int autofs4_ask_umount(struct vfsmount *mnt, int __user *p)
int is_autofs4_dentry(struct dentry *dentry)
{
return dentry && dentry->d_inode &&
- (dentry->d_op == &autofs4_root_dentry_operations ||
- dentry->d_op == &autofs4_dentry_operations) &&
+ (dentry->d_op == &autofs4_dentry_operations) &&
dentry->d_fsdata != NULL;
}
--
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