Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable

[Waiman Long]

On 05/12/2016 08:19 AM, Michal Hocko wrote:
> On Thu 12-05-16 14:12:04, Peter Zijlstra wrote:
> > On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote:
> > > I still cannot say I would understand why the pending
> > > RWSEM_WAITING_BIAS matters but I would probably need to look at the code
> > > again with a clean head, __rwsem_wake is quite tricky...
> > Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough?
> >
> > Because; if at that point there's nobody waiting, we're left with an
> > empty list and WAITER_BIAS set. This in turn will make all fast paths
> > fail.
> >
> > Look at rwsem_down_read_failed() for instance; if we enter that we'll
> > unconditionally queue ourself, with nobody left to come wake us.
> This is still not clear to me because rwsem_down_read_failed will call
> __rwsem_do_wake if the count is RWSEM_WAITING_BIAS so we shouldn't go to
> sleep and get the lock. So you are right that we would force everybody
> to the slow path which is not great but shouldn't cause incorrect
> behavior. I guess I must be missing something obvious here...

Because of writer lock stealing, having a count of RWSEM_WAITING_BIAS 
doesn't mean the reader can surely get the lock even if it is the first 
one in the queue. Calling __rwsem_do_wake() will take care of all the 
locking and queue checking work. Yes, I think it is a bit odd for the 
possibility that a task may wake up itself. Maybe we can add code like:

--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -202,7 +202,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum 
rwsem_wake_type wake_type)
          */
         smp_mb();
         waiter->task = NULL;
-        wake_up_process(tsk);
+        if (tsk != current)
+            wake_up_process(tsk);
         put_task_struct(tsk);
     } while (--loop);

Cheers,
Longman