Bug#1071501: Linux NFS client hangs in nfs4_lookup_revalidate

[Richard Kojedzinszky <richard+debian+bugreport@kojedz.in>]

Dear Neil,

I've stripped the code more, which still triggers the bug for me. On 
Bookworm, to get the binary, simply:

$ sudo apt-get install golang
$ go build .

And then give it an nfs mountpoint, e.g.:

$ ./ds /mnt/nfs

Meanwhile, I will try your patch too.

Regards,
Richard

2024-05-24 01:31 időpontban NeilBrown ezt írta:
> On Fri, 24 May 2024, Richard Kojedzinszky wrote:
> > Dear devs,
> >
> > I am attaching a stripped down version of the little program which
> > triggers the bug very quickly, in a few minutes in my test lab. It
> > turned out that a single NFS mountpoint is enough. Just start the
> > program giving it the NFS mount as first argument. It will chdir 
> > there,
> > 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 != NFS_FSDATA_BLOCKED);
> +			       smp_load_acquire(&dentry->d_fsdata) != NFS_FSDATA_BLOCKED);
>  		parent = dget_parent(dentry);
>  		ret = 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 = nfs_safe_remove(dentry);
>  	nfs_dentry_remove_handle_error(dir, dentry, error);
> -	dentry->d_fsdata = 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_renamedata *data)
>  {
>  	struct dentry *new_dentry = data->new_dentry;
>
> -	new_dentry->d_fsdata = NULL;
> +	smp_store_release(&new_dentry->d_fsdata, NULL);
>  	wake_up_var(&new_dentry->d_fsdata);
>  }
>
> @@ -2717,6 +2717,10 @@ int nfs_rename(struct mnt_idmap *idmap, struct 
> inode *old_dir,
>  	task = 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 = PTR_ERR(task);
>  		goto out;
>  	}

Attachment: ds.tar
Description: Unix tar archive