acct: get rid of acct_lock for acct->count

* make acct->count atomic and acct freeing - rcu-delayed.
* instead of grabbing acct_lock around the places where we take a reference,
do that under rcu_read_lock() with atomic_long_inc_not_zero().
* have the new acct locked before making ns->bacct point to it

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/kernel/acct.c b/kernel/acct.c
index 21fbb3c27c2adf858cb84d00258b17f4e1494a71..6fd375f1562696ac7774079e43b9737bcddf5c45 100644 (file)
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -79,9 +79,14 @@ int acct_parm[3] = {4, 2, 30};
 static void do_acct_process(struct bsd_acct_struct *acct);
 
 struct bsd_acct_struct {
-       long                    count;
-       struct hlist_node       s_list;
-       struct hlist_node       m_list;
+       atomic_long_t           count;
+       union {
+               struct {
+                       struct hlist_node       s_list;
+                       struct hlist_node       m_list;
+               };
+               struct rcu_head rcu;
+       };
        struct mutex            lock;
        int                     active;
        unsigned long           needcheck;
@@ -89,6 +94,11 @@ struct bsd_acct_struct {
        struct pid_namespace    *ns;
 };
 
+static void acct_free_rcu(struct rcu_head *head)
+{
+       kfree(container_of(head, struct bsd_acct_struct, rcu));
+}
+
 static DEFINE_SPINLOCK(acct_lock);
 
 /*
@@ -128,22 +138,22 @@ out:
 
 static void acct_put(struct bsd_acct_struct *p)
 {
-       spin_lock(&acct_lock);
-       if (!--p->count)
-               kfree(p);
-       spin_unlock(&acct_lock);
+       if (atomic_long_dec_and_test(&p->count))
+               call_rcu(&p->rcu, acct_free_rcu);
 }
 
 static struct bsd_acct_struct *__acct_get(struct bsd_acct_struct *res)
 {
-       res->count++;
-       spin_unlock(&acct_lock);
+       if (!atomic_long_inc_not_zero(&res->count)) {
+               rcu_read_unlock();
+               cpu_relax();
+               return NULL;
+       }
+       rcu_read_unlock();
        mutex_lock(&res->lock);
        if (!res->ns) {
                mutex_unlock(&res->lock);
-               spin_lock(&acct_lock);
-               if (!--res->count)
-                       kfree(res);
+               acct_put(res);
                return NULL;
        }
        return res;
@@ -152,13 +162,15 @@ static struct bsd_acct_struct *__acct_get(struct bsd_acct_struct *res)
 static struct bsd_acct_struct *acct_get(struct pid_namespace *ns)
 {
        struct bsd_acct_struct *res;
-       spin_lock(&acct_lock);
 again:
-       if (!ns->bacct) {
-               spin_unlock(&acct_lock);
+       smp_rmb();
+       rcu_read_lock();
+       res = ACCESS_ONCE(ns->bacct);
+       if (!res) {
+               rcu_read_unlock();
                return NULL;
        }
-       res = __acct_get(ns->bacct);
+       res = __acct_get(res);
        if (!res)
                goto again;
        return res;
@@ -170,26 +182,27 @@ static void acct_kill(struct bsd_acct_struct *acct,
        if (acct) {
                struct file *file = acct->file;
                struct pid_namespace *ns = acct->ns;
+               do_acct_process(acct);
+               mnt_unpin(file->f_path.mnt);
+               filp_close(file, NULL);
                spin_lock(&acct_lock);
                hlist_del(&acct->m_list);
                hlist_del(&acct->s_list);
-               mnt_unpin(file->f_path.mnt);
                spin_unlock(&acct_lock);
-               do_acct_process(acct);
-               filp_close(file, NULL);
-               spin_lock(&acct_lock);
                ns->bacct = new;
                if (new) {
                        struct vfsmount *m = new->file->f_path.mnt;
                        mnt_pin(m);
+                       spin_lock(&acct_lock);
                        hlist_add_head(&new->s_list, &m->mnt_sb->s_pins);
                        hlist_add_head(&new->m_list, &real_mount(m)->mnt_pins);
+                       spin_unlock(&acct_lock);
+                       mutex_unlock(&new->lock);
                }
                acct->ns = NULL;
+               atomic_long_dec(&acct->count);
                mutex_unlock(&acct->lock);
-               if (!(acct->count -= 2))
-                       kfree(acct);
-               spin_unlock(&acct_lock);
+               acct_put(acct);
        }
 }
 
@@ -223,7 +236,7 @@ static int acct_on(struct filename *pathname)
                return -EIO;
        }
 
-       acct->count = 1;
+       atomic_long_set(&acct->count, 1);
        acct->file = file;
        acct->needcheck = jiffies;
        acct->ns = ns;
@@ -231,15 +244,17 @@ static int acct_on(struct filename *pathname)
        mnt = file->f_path.mnt;
 
        old = acct_get(ns);
+       mutex_lock_nested(&acct->lock, 1);      /* nobody has seen it yet */
        if (old) {
                acct_kill(old, acct);
        } else {
-               spin_lock(&acct_lock);
                ns->bacct = acct;
+               spin_lock(&acct_lock);
                mnt_pin(mnt);
                hlist_add_head(&acct->s_list, &mnt->mnt_sb->s_pins);
                hlist_add_head(&acct->m_list, &real_mount(mnt)->mnt_pins);
                spin_unlock(&acct_lock);
+               mutex_unlock(&acct->lock);
        }
        mntput(mnt); /* it's pinned, now give up active reference */
        return 0;
@@ -282,28 +297,32 @@ SYSCALL_DEFINE1(acct, const char __user *, name)
 
 void acct_auto_close_mnt(struct hlist_head *list)
 {
+       rcu_read_lock();
        while (1) {
-               spin_lock(&acct_lock);
-               if (!list->first)
+               struct hlist_node *p = ACCESS_ONCE(list->first);
+               if (!p)
                        break;
-               acct_kill(__acct_get(hlist_entry(list->first,
+               acct_kill(__acct_get(hlist_entry(p,
                                                 struct bsd_acct_struct,
                                                 m_list)), NULL);
+               rcu_read_lock();
        }
-       spin_unlock(&acct_lock);
+       rcu_read_unlock();
 }
 
 void acct_auto_close(struct hlist_head *list)
 {
+       rcu_read_lock();
        while (1) {
-               spin_lock(&acct_lock);
-               if (!list->first)
+               struct hlist_node *p = ACCESS_ONCE(list->first);
+               if (!p)
                        break;
-               acct_kill(__acct_get(hlist_entry(list->first,
+               acct_kill(__acct_get(hlist_entry(p,
                                                 struct bsd_acct_struct,
                                                 s_list)), NULL);
+               rcu_read_lock();
        }
-       spin_unlock(&acct_lock);
+       rcu_read_unlock();
 }
 
 void acct_exit_ns(struct pid_namespace *ns)