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>

1 files changed, 48 insertions, 47 deletions

diff --git a/kernel/task_work.c b/kernel/task_work.c
index d320d44903b..f13ec0bda1d 100644
--- 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);
         }
 }