task_work: Make task_work_add() lockless

Change task_work's to use llist-like code to avoid pi_lock
in task_work_add(), this makes it useable under rq->lock.

task_work_cancel() and task_work_run() still use pi_lock
to synchronize with each other.

(This is in preparation for a deadlock fix.)

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120826191209.GA4221@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/task_work.c b/kernel/task_work.c
index d320d44903bd0ab8ce3a1dc3e40dee28a26e714f..f13ec0bda1d5af70713d5d237612d120141e84dd 100644 (file)
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -3,25 +3,18 @@
 #include <linux/tracehook.h>
 
 int
-task_work_add(struct task_struct *task, struct callback_head *twork, bool notify)
+task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 {
-       struct callback_head *last, *first;
-       unsigned long flags;
-
+       struct callback_head *head;
        /*
         * Not inserting the new work if the task has already passed
         * exit_task_work() is the responisbility of callers.
         */
-       raw_spin_lock_irqsave(&task->pi_lock, flags);
-       last = task->task_works;
-       first = last ? last->next : twork;
-       twork->next = first;
-       if (last)
-               last->next = twork;
-       task->task_works = twork;
-       raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+       do {
+               head = ACCESS_ONCE(task->task_works);
+               work->next = head;
+       } while (cmpxchg(&task->task_works, head, work) != head);
 
-       /* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
        if (notify)
                set_notify_resume(task);
        return 0;
@@ -30,52 +23,60 @@ task_work_add(struct task_struct *task, struct callback_head *twork, bool notify
 struct callback_head *
 task_work_cancel(struct task_struct *task, task_work_func_t func)
 {
+       struct callback_head **pprev = &task->task_works;
+       struct callback_head *work = NULL;
        unsigned long flags;
-       struct callback_head *last, *res = NULL;
-
+       /*
+        * If cmpxchg() fails we continue without updating pprev.
+        * Either we raced with task_work_add() which added the
+        * new entry before this work, we will find it again. Or
+        * we raced with task_work_run(), *pprev == NULL.
+        */
        raw_spin_lock_irqsave(&task->pi_lock, flags);
-       last = task->task_works;
-       if (last) {
-               struct callback_head *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;
-               }
+       while ((work = ACCESS_ONCE(*pprev))) {
+               read_barrier_depends();
+               if (work->func != func)
+                       pprev = &work->next;
+               else if (cmpxchg(pprev, work, work->next) == work)
+                       break;
        }
        raw_spin_unlock_irqrestore(&task->pi_lock, flags);
-       return res;
+
+       return work;
 }
 
 void task_work_run(void)
 {
        struct task_struct *task = current;
-       struct callback_head *p, *q;
+       struct callback_head *work, *head, *next;
 
-       while (1) {
-               raw_spin_lock_irq(&task->pi_lock);
-               p = task->task_works;
-               task->task_works = NULL;
-               raw_spin_unlock_irq(&task->pi_lock);
+       for (;;) {
+               work = xchg(&task->task_works, NULL);
+               if (!work)
+                       break;
+               /*
+                * Synchronize with task_work_cancel(). It can't remove
+                * the first entry == work, cmpxchg(task_works) should
+                * fail, but it can play with *work and other entries.
+                */
+               raw_spin_unlock_wait(&task->pi_lock);
+               smp_mb();
 
-               if (unlikely(!p))
-                       return;
+               /* Reverse the list to run the works in fifo order */
+               head = NULL;
+               do {
+                       next = work->next;
+                       work->next = head;
+                       head = work;
+                       work = next;
+               } while (work);
 
-               q = p->next; /* head */
-               p->next = NULL; /* cut it */
-               while (q) {
-                       p = q->next;
-                       q->func(q);
-                       q = p;
+               work = head;
+               do {
+                       next = work->next;
+                       work->func(work);
+                       work = next;
                        cond_resched();
-               }
+               } while (work);
        }
 }