[PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock

From: Minchan Kim
Date: Thu Nov 18 2021 - 18:00:21 EST

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Next in thread: Tejun Heo: "Re: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock"
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The kernfs implementation has big lock granularity(kernfs_rwsem) so
every kernfs-based(e.g., sysfs, cgroup) fs are able to compete the
lock. It makes trouble for some cases to wait the global lock
for a long time even though they are totally independent contexts
each other.

A general example is process A goes under direct reclaim with holding
the lock when it accessed the file in sysfs and process B is waiting
the lock with exclusive mode and then process C is waiting the lock
until process B could finish the job after it gets the lock from
process A.

This patch switches the global kernfs_rwsem to per-fs lock, which
put the rwsem into kernfs_root.

Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
from RFC - https://lore.kernel.org/lkml/20211116194317.1430399-1-minchan@xxxxxxxxxx/
* move kernfs_rwsem to kernfs_root - tejun@

fs/kernfs/dir.c | 110 ++++++++++++++++++++++++-----------------
fs/kernfs/file.c | 6 ++-
fs/kernfs/inode.c | 22 ++++++---
fs/kernfs/mount.c | 15 +++---
fs/kernfs/symlink.c | 5 +-
include/linux/kernfs.h | 2 +
6 files changed, 97 insertions(+), 63 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8e0a1378a4b1..13cae0ccce74 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -17,7 +17,6 @@

#include "kernfs-internal.h"

-DECLARE_RWSEM(kernfs_rwsem);
static DEFINE_SPINLOCK(kernfs_rename_lock); /* kn->parent and ->name */
static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by rename_lock */
static DEFINE_SPINLOCK(kernfs_idr_lock); /* root->ino_idr */
@@ -26,7 +25,7 @@ static DEFINE_SPINLOCK(kernfs_idr_lock); /* root->ino_idr */

static bool kernfs_active(struct kernfs_node *kn)
{
- lockdep_assert_held(&kernfs_rwsem);
+ lockdep_assert_held(&kernfs_root(kn)->kernfs_rwsem);
return atomic_read(&kn->active) >= 0;
}

@@ -457,14 +456,15 @@ void kernfs_put_active(struct kernfs_node *kn)
* return after draining is complete.
*/
static void kernfs_drain(struct kernfs_node *kn)
- __releases(&kernfs_rwsem) __acquires(&kernfs_rwsem)
+ __releases(&kernfs_root(kn)->kernfs_rwsem)
+ __acquires(&kernfs_root(kn)->kernfs_rwsem)
{
struct kernfs_root *root = kernfs_root(kn);

- lockdep_assert_held_write(&kernfs_rwsem);
+ lockdep_assert_held_write(&root->kernfs_rwsem);
WARN_ON_ONCE(kernfs_active(kn));

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);

if (kernfs_lockdep(kn)) {
rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_);
@@ -483,7 +483,7 @@ static void kernfs_drain(struct kernfs_node *kn)

kernfs_drain_open_files(kn);

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
}

/**
@@ -718,11 +718,12 @@ struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
int kernfs_add_one(struct kernfs_node *kn)
{
struct kernfs_node *parent = kn->parent;
+ struct kernfs_root *root = kernfs_root(parent);
struct kernfs_iattrs *ps_iattr;
bool has_ns;
int ret;

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);

ret = -EINVAL;
has_ns = kernfs_ns_enabled(parent);
@@ -753,7 +754,7 @@ int kernfs_add_one(struct kernfs_node *kn)
ps_iattr->ia_mtime = ps_iattr->ia_ctime;
}

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);

/*
* Activate the new node unless CREATE_DEACTIVATED is requested.
@@ -767,7 +768,7 @@ int kernfs_add_one(struct kernfs_node *kn)
return 0;

out_unlock:
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
return ret;
}

@@ -788,7 +789,7 @@ static struct kernfs_node *kernfs_find_ns(struct kernfs_node *parent,
bool has_ns = kernfs_ns_enabled(parent);
unsigned int hash;

- lockdep_assert_held(&kernfs_rwsem);
+ lockdep_assert_held(&kernfs_root(parent)->kernfs_rwsem);

if (has_ns != (bool)ns) {
WARN(1, KERN_WARNING "kernfs: ns %s in '%s' for '%s'\n",
@@ -820,7 +821,7 @@ static struct kernfs_node *kernfs_walk_ns(struct kernfs_node *parent,
size_t len;
char *p, *name;

- lockdep_assert_held_read(&kernfs_rwsem);
+ lockdep_assert_held_read(&kernfs_root(parent)->kernfs_rwsem);

/* grab kernfs_rename_lock to piggy back on kernfs_pr_cont_buf */
spin_lock_irq(&kernfs_rename_lock);
@@ -859,11 +860,12 @@ struct kernfs_node *kernfs_find_and_get_ns(struct kernfs_node *parent,
const char *name, const void *ns)
{
struct kernfs_node *kn;
+ struct kernfs_root *root = kernfs_root(parent);

- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
kn = kernfs_find_ns(parent, name, ns);
kernfs_get(kn);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

return kn;
}
@@ -883,11 +885,12 @@ struct kernfs_node *kernfs_walk_and_get_ns(struct kernfs_node *parent,
const char *path, const void *ns)
{
struct kernfs_node *kn;
+ struct kernfs_root *root = kernfs_root(parent);

- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
kn = kernfs_walk_ns(parent, path, ns);
kernfs_get(kn);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

return kn;
}
@@ -912,6 +915,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
return ERR_PTR(-ENOMEM);

idr_init(&root->ino_idr);
+ init_rwsem(&root->kernfs_rwsem);
INIT_LIST_HEAD(&root->supers);

/*
@@ -1035,6 +1039,7 @@ struct kernfs_node *kernfs_create_empty_dir(struct kernfs_node *parent,
static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
{
struct kernfs_node *kn;
+ struct kernfs_root *root;

if (flags & LOOKUP_RCU)
return -ECHILD;
@@ -1046,18 +1051,19 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
/* If the kernfs parent node has changed discard and
* proceed to ->lookup.
*/
- down_read(&kernfs_rwsem);
spin_lock(&dentry->d_lock);
parent = kernfs_dentry_node(dentry->d_parent);
if (parent) {
+ spin_unlock(&dentry->d_lock);
+ root = kernfs_root(parent);
+ down_read(&root->kernfs_rwsem);
if (kernfs_dir_changed(parent, dentry)) {
- spin_unlock(&dentry->d_lock);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
return 0;
}
- }
- spin_unlock(&dentry->d_lock);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
+ } else
+ spin_unlock(&dentry->d_lock);

/* The kernfs parent node hasn't changed, leave the
* dentry negative and return success.
@@ -1066,7 +1072,8 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
}

kn = kernfs_dentry_node(dentry);
- down_read(&kernfs_rwsem);
+ root = kernfs_root(kn);
+ down_read(&root->kernfs_rwsem);

/* The kernfs node has been deactivated */
if (!kernfs_active(kn))
@@ -1085,10 +1092,10 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
kernfs_info(dentry->d_sb)->ns != kn->ns)
goto out_bad;

- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
return 1;
out_bad:
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
return 0;
}

@@ -1102,10 +1109,12 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
{
struct kernfs_node *parent = dir->i_private;
struct kernfs_node *kn;
+ struct kernfs_root *root;
struct inode *inode = NULL;
const void *ns = NULL;

- down_read(&kernfs_rwsem);
+ root = kernfs_root(parent);
+ down_read(&root->kernfs_rwsem);
if (kernfs_ns_enabled(parent))
ns = kernfs_info(dir->i_sb)->ns;

@@ -1116,7 +1125,7 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
* create a negative.
*/
if (!kernfs_active(kn)) {
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
return NULL;
}
inode = kernfs_get_inode(dir->i_sb, kn);
@@ -1131,7 +1140,7 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
*/
if (!IS_ERR(inode))
kernfs_set_rev(parent, dentry);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

/* instantiate and hash (possibly negative) dentry */
return d_splice_alias(inode, dentry);
@@ -1254,7 +1263,7 @@ static struct kernfs_node *kernfs_next_descendant_post(struct kernfs_node *pos,
{
struct rb_node *rbn;

- lockdep_assert_held_write(&kernfs_rwsem);
+ lockdep_assert_held_write(&kernfs_root(root)->kernfs_rwsem);

/* if first iteration, visit leftmost descendant which may be root */
if (!pos)
@@ -1289,8 +1298,9 @@ static struct kernfs_node *kernfs_next_descendant_post(struct kernfs_node *pos,
void kernfs_activate(struct kernfs_node *kn)
{
struct kernfs_node *pos;
+ struct kernfs_root *root = kernfs_root(kn);

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);

pos = NULL;
while ((pos = kernfs_next_descendant_post(pos, kn))) {
@@ -1304,14 +1314,14 @@ void kernfs_activate(struct kernfs_node *kn)
pos->flags |= KERNFS_ACTIVATED;
}

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
}

static void __kernfs_remove(struct kernfs_node *kn)
{
struct kernfs_node *pos;

- lockdep_assert_held_write(&kernfs_rwsem);
+ lockdep_assert_held_write(&kernfs_root(kn)->kernfs_rwsem);

/*
* Short-circuit if non-root @kn has already finished removal.
@@ -1381,9 +1391,11 @@ static void __kernfs_remove(struct kernfs_node *kn)
*/
void kernfs_remove(struct kernfs_node *kn)
{
- down_write(&kernfs_rwsem);
+ struct kernfs_root *root = kernfs_root(kn);
+
+ down_write(&root->kernfs_rwsem);
__kernfs_remove(kn);
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
}

/**
@@ -1469,8 +1481,9 @@ void kernfs_unbreak_active_protection(struct kernfs_node *kn)
bool kernfs_remove_self(struct kernfs_node *kn)
{
bool ret;
+ struct kernfs_root *root = kernfs_root(kn);

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
kernfs_break_active_protection(kn);

/*
@@ -1498,9 +1511,9 @@ bool kernfs_remove_self(struct kernfs_node *kn)
atomic_read(&kn->active) == KN_DEACTIVATED_BIAS)
break;

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
schedule();
- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
}
finish_wait(waitq, &wait);
WARN_ON_ONCE(!RB_EMPTY_NODE(&kn->rb));
@@ -1513,7 +1526,7 @@ bool kernfs_remove_self(struct kernfs_node *kn)
*/
kernfs_unbreak_active_protection(kn);

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
return ret;
}

@@ -1530,6 +1543,7 @@ int kernfs_remove_by_name_ns(struct kernfs_node *parent, const char *name,
const void *ns)
{
struct kernfs_node *kn;
+ struct kernfs_root *root;

if (!parent) {
WARN(1, KERN_WARNING "kernfs: can not remove '%s', no directory\n",
@@ -1537,13 +1551,14 @@ int kernfs_remove_by_name_ns(struct kernfs_node *parent, const char *name,
return -ENOENT;
}

- down_write(&kernfs_rwsem);
+ root = kernfs_root(parent);
+ down_write(&root->kernfs_rwsem);

kn = kernfs_find_ns(parent, name, ns);
if (kn)
__kernfs_remove(kn);

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);

if (kn)
return 0;
@@ -1562,6 +1577,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
const char *new_name, const void *new_ns)
{
struct kernfs_node *old_parent;
+ struct kernfs_root *root;
const char *old_name = NULL;
int error;

@@ -1569,7 +1585,8 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
if (!kn->parent)
return -EINVAL;

- down_write(&kernfs_rwsem);
+ root = kernfs_root(kn);
+ down_write(&root->kernfs_rwsem);

error = -ENOENT;
if (!kernfs_active(kn) || !kernfs_active(new_parent) ||
@@ -1623,7 +1640,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,

error = 0;
out:
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
return error;
}

@@ -1694,11 +1711,14 @@ static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
struct dentry *dentry = file->f_path.dentry;
struct kernfs_node *parent = kernfs_dentry_node(dentry);
struct kernfs_node *pos = file->private_data;
+ struct kernfs_root *root;
const void *ns = NULL;

if (!dir_emit_dots(file, ctx))
return 0;
- down_read(&kernfs_rwsem);
+
+ root = kernfs_root(parent);
+ down_read(&root->kernfs_rwsem);

if (kernfs_ns_enabled(parent))
ns = kernfs_info(dentry->d_sb)->ns;
@@ -1715,12 +1735,12 @@ static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
file->private_data = pos;
kernfs_get(pos);

- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
if (!dir_emit(ctx, name, len, ino, type))
return 0;
- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
}
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);
file->private_data = NULL;
ctx->pos = INT_MAX;
return 0;
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 60e2a86c535e..9414a7a60a9f 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -847,6 +847,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
{
struct kernfs_node *kn;
struct kernfs_super_info *info;
+ struct kernfs_root *root;
repeat:
/* pop one off the notify_list */
spin_lock_irq(&kernfs_notify_lock);
@@ -859,8 +860,9 @@ static void kernfs_notify_workfn(struct work_struct *work)
kn->attr.notify_next = NULL;
spin_unlock_irq(&kernfs_notify_lock);

+ root = kernfs_root(kn);
/* kick fsnotify */
- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);

list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
struct kernfs_node *parent;
@@ -898,7 +900,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
iput(inode);
}

- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
kernfs_put(kn);
goto repeat;
}
diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index c0eae1725435..3d783d80f5da 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -99,10 +99,11 @@ int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
int kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
{
int ret;
+ struct kernfs_root *root = kernfs_root(kn);

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
ret = __kernfs_setattr(kn, iattr);
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
return ret;
}

@@ -111,12 +112,14 @@ int kernfs_iop_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
{
struct inode *inode = d_inode(dentry);
struct kernfs_node *kn = inode->i_private;
+ struct kernfs_root *root;
int error;

if (!kn)
return -EINVAL;

- down_write(&kernfs_rwsem);
+ root = kernfs_root(kn);
+ down_write(&root->kernfs_rwsem);
error = setattr_prepare(&init_user_ns, dentry, iattr);
if (error)
goto out;
@@ -129,7 +132,7 @@ int kernfs_iop_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
setattr_copy(&init_user_ns, inode, iattr);

out:
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
return error;
}

@@ -184,13 +187,14 @@ int kernfs_iop_getattr(struct user_namespace *mnt_userns,
{
struct inode *inode = d_inode(path->dentry);
struct kernfs_node *kn = inode->i_private;
+ struct kernfs_root *root = kernfs_root(kn);

- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
spin_lock(&inode->i_lock);
kernfs_refresh_inode(kn, inode);
generic_fillattr(&init_user_ns, inode, stat);
spin_unlock(&inode->i_lock);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

return 0;
}
@@ -274,19 +278,21 @@ int kernfs_iop_permission(struct user_namespace *mnt_userns,
struct inode *inode, int mask)
{
struct kernfs_node *kn;
+ struct kernfs_root *root;
int ret;

if (mask & MAY_NOT_BLOCK)
return -ECHILD;

kn = inode->i_private;
+ root = kernfs_root(kn);

- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
spin_lock(&inode->i_lock);
kernfs_refresh_inode(kn, inode);
ret = generic_permission(&init_user_ns, inode, mask);
spin_unlock(&inode->i_lock);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

return ret;
}
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index f2f909d09f52..cfa79715fc1a 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -236,6 +236,7 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
static int kernfs_fill_super(struct super_block *sb, struct kernfs_fs_context *kfc)
{
struct kernfs_super_info *info = kernfs_info(sb);
+ struct kernfs_root *kf_root = kfc->root;
struct inode *inode;
struct dentry *root;

@@ -255,9 +256,9 @@ static int kernfs_fill_super(struct super_block *sb, struct kernfs_fs_context *k
sb->s_shrink.seeks = 0;

/* get root inode, initialize and unlock it */
- down_read(&kernfs_rwsem);
+ down_read(&kf_root->kernfs_rwsem);
inode = kernfs_get_inode(sb, info->root->kn);
- up_read(&kernfs_rwsem);
+ up_read(&kf_root->kernfs_rwsem);
if (!inode) {
pr_debug("kernfs: could not get root inode\n");
return -ENOMEM;
@@ -334,6 +335,7 @@ int kernfs_get_tree(struct fs_context *fc)

if (!sb->s_root) {
struct kernfs_super_info *info = kernfs_info(sb);
+ struct kernfs_root *root = kfc->root;

kfc->new_sb_created = true;

@@ -344,9 +346,9 @@ int kernfs_get_tree(struct fs_context *fc)
}
sb->s_flags |= SB_ACTIVE;

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
list_add(&info->node, &info->root->supers);
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);
}

fc->root = dget(sb->s_root);
@@ -371,10 +373,11 @@ void kernfs_free_fs_context(struct fs_context *fc)
void kernfs_kill_sb(struct super_block *sb)
{
struct kernfs_super_info *info = kernfs_info(sb);
+ struct kernfs_root *root = info->root;

- down_write(&kernfs_rwsem);
+ down_write(&root->kernfs_rwsem);
list_del(&info->node);
- up_write(&kernfs_rwsem);
+ up_write(&root->kernfs_rwsem);

/*
* Remove the superblock from fs_supers/s_instances
diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 19a6c71c6ff5..0ab13824822f 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -113,11 +113,12 @@ static int kernfs_getlink(struct inode *inode, char *path)
struct kernfs_node *kn = inode->i_private;
struct kernfs_node *parent = kn->parent;
struct kernfs_node *target = kn->symlink.target_kn;
+ struct kernfs_root *root = kernfs_root(parent);
int error;

- down_read(&kernfs_rwsem);
+ down_read(&root->kernfs_rwsem);
error = kernfs_get_target_path(parent, target, path);
- up_read(&kernfs_rwsem);
+ up_read(&root->kernfs_rwsem);

return error;
}
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 3ccce6f24548..9f650986a81b 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -16,6 +16,7 @@
#include <linux/atomic.h>
#include <linux/uidgid.h>
#include <linux/wait.h>
+#include <linux/rwsem.h>

struct file;
struct dentry;
@@ -197,6 +198,7 @@ struct kernfs_root {
struct list_head supers;

wait_queue_head_t deactivate_waitq;
+ struct rw_semaphore kernfs_rwsem;
};

struct kernfs_open_file {
--
2.34.0.rc2.393.gf8c9666880-goog

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