On Thu, Jun 19, 2025 at 12:01:19PM +0200, Christian Brauner wrote:

On Wed, Jun 18, 2025 at 04:37:36PM -0700, Song Liu wrote:

quoted

Existing kernfs_xattr_get() locks iattr_mutex, so it cannot be used in
RCU critical sections. Introduce __kernfs_xattr_get(), which reads xattr
under RCU read lock. This can be used by BPF programs to access cgroupfs
xattrs.

Signed-off-by: Song Liu <song@kernel.org>
---
 fs/kernfs/inode.c      | 14 ++++++++++++++
 include/linux/kernfs.h |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index b83054da68b3..0ca231d2012c 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c

@@ -302,6 +302,20 @@ int kernfs_xattr_get(struct kernfs_node *kn, const char *name,
 	return simple_xattr_get(&attrs->xattrs, name, value, size);
 }
 
+int __kernfs_xattr_get(struct kernfs_node *kn, const char *name,
+		       void *value, size_t size)
+{
+	struct kernfs_iattrs *attrs;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	attrs = rcu_dereference(kn->iattr);
+	if (!attrs)
+		return -ENODATA;

Hm, that looks a bit silly. Which isn't your fault. I'm looking at the
kernfs code that does the xattr allocations and I think that's the
origin of the silliness. It uses a single global mutex for all kernfs
users thus serializing all allocations for kernfs->iattr. That seems
crazy but maybe I'm missing a good reason.

I'm appending a patch to remove that mutex. @Greg, @Tejun, can you take
a look whether that makes sense to you. Then I can take that patch and
you can build yours on top of the series and I'll pick it all up in one
go.

Looks sane to me, thanks!

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>