trim task_work: get rid of hlist

layout based on Oleg's suggestion; single-linked list,
task->task_works points to the last element, forward pointer
from said last element points to head.  I'd still prefer
much more regular scheme with two pointers in task_work,
but...

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

5 files changed, 38 insertions, 36 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4059c0f33f07..b9216ebc2789 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1405,7 +1405,7 @@ struct task_struct {
         int (*notifier)(void *priv);
         void *notifier_data;
         sigset_t *notifier_mask;
-        struct hlist_head task_works;
+        void *task_works;
 
         struct audit_context *audit_context;
 #ifdef CONFIG_AUDITSYSCALL
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 627421c0e108..3b3e2c8d037b 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -8,7 +8,7 @@ struct task_work;
 typedef void (*task_work_func_t)(struct task_work *);
 
 struct task_work {
-        struct hlist_node hlist;
+        struct task_work *next;
         task_work_func_t func;
 };
 
@@ -24,7 +24,7 @@ void task_work_run(void);
 
 static inline void exit_task_work(struct task_struct *task)
 {
-        if (unlikely(!hlist_empty(&task->task_works)))
+        if (unlikely(task->task_works))
                 task_work_run();
 }
 
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 6a4d82bedb03..1e98b5530425 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -192,7 +192,7 @@ static inline void tracehook_notify_resume(struct pt_regs *regs)
          * hlist_add_head(task->task_works);
          */
         smp_mb__after_clear_bit();
-        if (unlikely(!hlist_empty(&current->task_works)))
+        if (unlikely(current->task_works))
                 task_work_run();
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index ab5211b9e622..bebabad59202 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1415,7 +1415,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
          */
         p->group_leader = p;
         INIT_LIST_HEAD(&p->thread_group);
-        INIT_HLIST_HEAD(&p->task_works);
+        p->task_works = NULL;
 
         /* Now that the task is set up, run cgroup callbacks if
          * necessary. We need to run them before the task is visible
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 82d1c794066d..9b8948dbdc60 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -19,7 +19,12 @@ task_work_add(struct task_struct *task, struct task_work *twork, bool notify)
          */
         raw_spin_lock_irqsave(&task->pi_lock, flags);
         if (likely(!(task->flags & PF_EXITING))) {
-                hlist_add_head(&twork->hlist, &task->task_works);
+                struct task_work *last = task->task_works;
+                struct task_work *first = last ? last->next : twork;
+                twork->next = first;
+                if (last)
+                        last->next = twork;
+                task->task_works = twork;
                 err = 0;
         }
         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
@@ -34,51 +39,48 @@ struct task_work *
 task_work_cancel(struct task_struct *task, task_work_func_t func)
 {
         unsigned long flags;
-        struct task_work *twork;
-        struct hlist_node *pos;
+        struct task_work *last, *res = NULL;
 
         raw_spin_lock_irqsave(&task->pi_lock, flags);
-        hlist_for_each_entry(twork, pos, &task->task_works, hlist) {
-                if (twork->func == func) {
-                        hlist_del(&twork->hlist);
-                        goto found;
+        last = task->task_works;
+        if (last) {
+                struct task_work *q = last, *p = q->next;
+                while (1) {
+                        if (p->func == func) {
+                                q->next = p->next;
+                                if (p == last)
+                                        task->task_works = q == p ? NULL : q;
+                                res = p;
+                                break;
+                        }
+                        if (p == last)
+                                break;
+                        q = p;
+                        p = q->next;
                 }
         }
-        twork = NULL;
- found:
         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
-
-        return twork;
+        return res;
 }
 
 void task_work_run(void)
 {
         struct task_struct *task = current;
-        struct hlist_head task_works;
-        struct hlist_node *pos;
+        struct task_work *p, *q;
 
         raw_spin_lock_irq(&task->pi_lock);
-        hlist_move_list(&task->task_works, &task_works);
+        p = task->task_works;
+        task->task_works = NULL;
         raw_spin_unlock_irq(&task->pi_lock);
 
-        if (unlikely(hlist_empty(&task_works)))
+        if (unlikely(!p))
                 return;
-        /*
-         * We use hlist to save the space in task_struct, but we want fifo.
-         * Find the last entry, the list should be short, then process them
-         * in reverse order.
-         */
-        for (pos = task_works.first; pos->next; pos = pos->next)
-                ;
-
-        for (;;) {
-                struct hlist_node **pprev = pos->pprev;
-                struct task_work *twork = container_of(pos, struct task_work,
-                                                        hlist);
-                twork->func(twork);
 
-                if (pprev == &task_works.first)
-                        break;
-                pos = container_of(pprev, struct hlist_node, next);
+        q = p->next; /* head */
+        p->next = NULL; /* cut it */
+        while (q) {
+                p = q->next;
+                q->func(q);
+                q = p;
         }
 }