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>

1 files changed, 52 insertions, 33 deletions

diff --git a/kernel/acct.c b/kernel/acct.c
index 21fbb3c27c2a..6fd375f15626 100644
--- 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)