On Fri, Jun 30, 2017 at 01:02:48PM -0700, Paul E. McKenney wrote: > On Fri, Jun 30, 2017 at 09:21:23PM +0200, Oleg Nesterov wrote: > > On 06/30, Paul E. McKenney wrote: > > > > > > On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote: > > > > > > > > I do not think the overhead will be noticeable in this particular case. > > > > > > > > But I am not sure I understand why do we want to unlock_wait. Yes I agree, > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > > > if it was not clear, I tried to say "why do we want to _remove_ unlock_wait". > > > > > > it has some problems, but still... > > > > > > > > The code above looks strange for me. If we are going to repeat this pattern > > > > the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;) > > > > > > > > If not, we should probably change this code more: > > > > > > This looks -much- better than my patch! May I have your Signed-off-by? > > > > Only if you promise to replace all RCU flavors with a single simple implementation > > based on rwlock ;) > > ;-) ;-) ;-) > > Here you go: > > https://github.com/pramalhe/ConcurrencyFreaks/blob/master/papers/poormanurcu-2015.pdf > > > Seriously, of course I won't argue, and it seems that nobody except me likes > > this primitive, but to me spin_unlock_wait() looks like synchronize_rcu(() and > > sometimes it makes sense. > > Well, that analogy was what led me to propose that its semantics be > defined as spin_lock() immediately followed by spin_unlock(). But that > didn't go over well. > > > Including this particular case. task_work_run() is going to flush/destroy the > > ->task_works list, so it needs to wait until all currently executing "readers" > > (task_work_cancel()'s which have started before ->task_works was updated) have > > completed. > > Understood! And please see below for the resulting patch and commit log. Please let me know if I broke something. Thanx, Paul ------------------------------------------------------------------------ commit 6c0801c9ab19fc2f4c1e2436eb1b72e0af9a317b Author: Oleg Nesterov <oleg@xxxxxxxxxx> Date: Fri Jun 30 13:13:59 2017 -0700 task_work: Replace spin_unlock_wait() with lock/unlock pair There is no agreed-upon definition of spin_unlock_wait()'s semantics, and it appears that all callers could do just as well with a lock/unlock pair. This commit therefore replaces the spin_unlock_wait() call in task_work_run() with a spin_lock_irq() and a spin_unlock_irq() aruond the cmpxchg() dequeue loop. This should be safe from a performance perspective because ->pi_lock is local to the task and because calls to the other side of the race, task_work_cancel(), should be rare. Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> diff --git a/kernel/task_work.c b/kernel/task_work.c index d513051fcca2..836a72a66fba 100644 --- a/kernel/task_work.c +++ b/kernel/task_work.c @@ -96,20 +96,16 @@ void task_work_run(void) * work->func() can do task_work_add(), do not set * work_exited unless the list is empty. */ + raw_spin_lock_irq(&task->pi_lock); do { work = READ_ONCE(task->task_works); head = !work && (task->flags & PF_EXITING) ? &work_exited : NULL; } while (cmpxchg(&task->task_works, work, head) != work); + raw_spin_unlock_irq(&task->pi_lock); 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); do { next = work->next;
