kernfs: remove iattr_mutex

All allocations of struct kernfs_iattrs are serialized through a global
mutex. Simply do a racy allocation and let the first one win. I bet most
callers are under inode->i_rwsem anyway and it wouldn't be needed but
let's not require that.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/20250623063854.1896364-2-song@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index b83054da68b3e4992b75f6568dfa3acb923abc34..457f91c412d4835b1cffd7ad73a160c596730281 100644 (file)
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -24,45 +24,46 @@ static const struct inode_operations kernfs_iops = {
        .listxattr      = kernfs_iop_listxattr,
 };
 
-static struct kernfs_iattrs *__kernfs_iattrs(struct kernfs_node *kn, int alloc)
+static struct kernfs_iattrs *__kernfs_iattrs(struct kernfs_node *kn, bool alloc)
 {
-       static DEFINE_MUTEX(iattr_mutex);
-       struct kernfs_iattrs *ret;
+       struct kernfs_iattrs *ret __free(kfree) = NULL;
+       struct kernfs_iattrs *attr;
 
-       mutex_lock(&iattr_mutex);
+       attr = READ_ONCE(kn->iattr);
+       if (attr || !alloc)
+               return attr;
 
-       if (kn->iattr || !alloc)
-               goto out_unlock;
-
-       kn->iattr = kmem_cache_zalloc(kernfs_iattrs_cache, GFP_KERNEL);
-       if (!kn->iattr)
-               goto out_unlock;
+       ret = kmem_cache_zalloc(kernfs_iattrs_cache, GFP_KERNEL);
+       if (!ret)
+               return NULL;
 
        /* assign default attributes */
-       kn->iattr->ia_uid = GLOBAL_ROOT_UID;
-       kn->iattr->ia_gid = GLOBAL_ROOT_GID;
-
-       ktime_get_real_ts64(&kn->iattr->ia_atime);
-       kn->iattr->ia_mtime = kn->iattr->ia_atime;
-       kn->iattr->ia_ctime = kn->iattr->ia_atime;
-
-       simple_xattrs_init(&kn->iattr->xattrs);
-       atomic_set(&kn->iattr->nr_user_xattrs, 0);
-       atomic_set(&kn->iattr->user_xattr_size, 0);
-out_unlock:
-       ret = kn->iattr;
-       mutex_unlock(&iattr_mutex);
-       return ret;
+       ret->ia_uid = GLOBAL_ROOT_UID;
+       ret->ia_gid = GLOBAL_ROOT_GID;
+
+       ktime_get_real_ts64(&ret->ia_atime);
+       ret->ia_mtime = ret->ia_atime;
+       ret->ia_ctime = ret->ia_atime;
+
+       simple_xattrs_init(&ret->xattrs);
+       atomic_set(&ret->nr_user_xattrs, 0);
+       atomic_set(&ret->user_xattr_size, 0);
+
+       /* If someone raced us, recognize it. */
+       if (!try_cmpxchg(&kn->iattr, &attr, ret))
+               return READ_ONCE(kn->iattr);
+
+       return no_free_ptr(ret);
 }
 
 static struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn)
 {
-       return __kernfs_iattrs(kn, 1);
+       return __kernfs_iattrs(kn, true);
 }
 
 static struct kernfs_iattrs *kernfs_iattrs_noalloc(struct kernfs_node *kn)
 {
-       return __kernfs_iattrs(kn, 0);
+       return __kernfs_iattrs(kn, false);
 }
 
 int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
@@ -166,9 +167,10 @@ static inline void set_inode_attr(struct inode *inode,
 
 static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
 {
-       struct kernfs_iattrs *attrs = kn->iattr;
+       struct kernfs_iattrs *attrs;
 
        inode->i_mode = kn->mode;
+       attrs = kernfs_iattrs_noalloc(kn);
        if (attrs)
                /*
                 * kernfs_node has non-default attributes get them from
@@ -306,7 +308,9 @@ int kernfs_xattr_set(struct kernfs_node *kn, const char *name,
                     const void *value, size_t size, int flags)
 {
        struct simple_xattr *old_xattr;
-       struct kernfs_iattrs *attrs = kernfs_iattrs(kn);
+       struct kernfs_iattrs *attrs;
+
+       attrs = kernfs_iattrs(kn);
        if (!attrs)
                return -ENOMEM;
 
@@ -345,8 +349,9 @@ static int kernfs_vfs_user_xattr_add(struct kernfs_node *kn,
                                     struct simple_xattrs *xattrs,
                                     const void *value, size_t size, int flags)
 {
-       atomic_t *sz = &kn->iattr->user_xattr_size;
-       atomic_t *nr = &kn->iattr->nr_user_xattrs;
+       struct kernfs_iattrs *attr = kernfs_iattrs_noalloc(kn);
+       atomic_t *sz = &attr->user_xattr_size;
+       atomic_t *nr = &attr->nr_user_xattrs;
        struct simple_xattr *old_xattr;
        int ret;
 
@@ -384,8 +389,9 @@ static int kernfs_vfs_user_xattr_rm(struct kernfs_node *kn,
                                    struct simple_xattrs *xattrs,
                                    const void *value, size_t size, int flags)
 {
-       atomic_t *sz = &kn->iattr->user_xattr_size;
-       atomic_t *nr = &kn->iattr->nr_user_xattrs;
+       struct kernfs_iattrs *attr = kernfs_iattrs_noalloc(kn);
+       atomic_t *sz = &attr->user_xattr_size;
+       atomic_t *nr = &attr->nr_user_xattrs;
        struct simple_xattr *old_xattr;
 
        old_xattr = simple_xattr_set(xattrs, full_name, value, size, flags);