>From bdc53435a1cd5c456dc28d8239eff0e7fa4e8dda Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 19 Jun 2025 11:50:26 +0200 Subject: [PATCH] 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. Signed-off-by: Christian Brauner --- Note, that this uses kfree() for the kmem cache allocation. That's been possible for a while now but not everyone knows about it yet so I'm pointing it out explicitly. --- fs/kernfs/inode.c | 74 +++++++++++++++++++++++++---------------------- 1 file changed, 40 insertions(+), 34 deletions(-) diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c index b83054da68b3..f4b73b9482b7 100644 --- 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) @@ -141,9 +142,9 @@ ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size) struct kernfs_node *kn = kernfs_dentry_node(dentry); struct kernfs_iattrs *attrs; - attrs = kernfs_iattrs(kn); + attrs = kernfs_iattrs_noalloc(kn); if (!attrs) - return -ENOMEM; + return -ENODATA; return simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size); } @@ -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(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); -- 2.47.2