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From: "NeilBrown" <neilb@suse.de>
To: "Richard Kojedzinszky" <richard+debian+bugreport@kojedz.in>
Cc: linux-nfs@vger.kernel.org, 1071501@bugs.debian.org
Subject: Re: Linux NFS client hangs in nfs4_lookup_revalidate
In-reply-to: <162d12087ba8374a57e2263d7ea762b5@kojedz.in>
References: <>, <162d12087ba8374a57e2263d7ea762b5@kojedz.in>
Date: Fri, 24 May 2024 09:31:44 +1000
Message-id: <171650710476.27191.9102106000258626652@noble.neil.brown.name>

On Fri, 24 May 2024, Richard Kojedzinszky wrote:
> Dear devs,
>=20
> I am attaching a stripped down version of the little program which=20
> triggers the bug very quickly, in a few minutes in my test lab. It=20
> turned out that a single NFS mountpoint is enough. Just start the=20
> program giving it the NFS mount as first argument. It will chdir there,=20
> and do file operations, which will trigger a lockup in a few minutes.

I couldn't get the go code to run.  But then it is a long time since I
played with go and I didn't try very hard.
If you could provide simple instructions and a list of package
dependencies that I need to install (on Debian), I can give it a try.

Or you could try this patch.  It might help, but I don't have high
hopes.  It adds some memory barriers and fixes a bug which would cause a
problem if memory allocation failed (but memory allocation never fails).

NeilBrown

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index ac505671efbd..5bcc0d14d519 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1804,7 +1804,7 @@ __nfs_lookup_revalidate(struct dentry *dentry, unsigned=
 int flags,
 	} else {
 		/* Wait for unlink to complete */
 		wait_var_event(&dentry->d_fsdata,
-			       dentry->d_fsdata !=3D NFS_FSDATA_BLOCKED);
+			       smp_load_acquire(&dentry->d_fsdata) !=3D NFS_FSDATA_BLOCKED);
 		parent =3D dget_parent(dentry);
 		ret =3D reval(d_inode(parent), dentry, flags);
 		dput(parent);
@@ -2508,7 +2508,7 @@ int nfs_unlink(struct inode *dir, struct dentry *dentry)
 	spin_unlock(&dentry->d_lock);
 	error =3D nfs_safe_remove(dentry);
 	nfs_dentry_remove_handle_error(dir, dentry, error);
-	dentry->d_fsdata =3D NULL;
+	smp_store_release(&dentry->d_fsdata, NULL);
 	wake_up_var(&dentry->d_fsdata);
 out:
 	trace_nfs_unlink_exit(dir, dentry, error);
@@ -2616,7 +2616,7 @@ nfs_unblock_rename(struct rpc_task *task, struct nfs_re=
namedata *data)
 {
 	struct dentry *new_dentry =3D data->new_dentry;
=20
-	new_dentry->d_fsdata =3D NULL;
+	smp_store_release(&new_dentry->d_fsdata, NULL);
 	wake_up_var(&new_dentry->d_fsdata);
 }
=20
@@ -2717,6 +2717,10 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *=
old_dir,
 	task =3D nfs_async_rename(old_dir, new_dir, old_dentry, new_dentry,
 				must_unblock ? nfs_unblock_rename : NULL);
 	if (IS_ERR(task)) {
+		if (must_unlock) {
+			smp_store_release(&new_dentry->d_fsdata, NULL);
+			wake_up_var(&new_dentry->d_fsdata);
+		}
 		error =3D PTR_ERR(task);
 		goto out;
 	}