Re: [PATCH v3 07/13] futex: Throughput-optimized (TP) futexes

[Waiman Long]

On 10/01/2016 02:47 AM, Thomas Gleixner wrote:
> On Fri, 30 Sep 2016, Waiman Long wrote:
> > +	WRITE_ONCE(state->owner, current);
> > +	preempt_disable();
> > +	for (;;) {
> > +		ret = futex_trylock(uaddr, vpid,&uval, true);
> Did you actually read what I said? You CANNOT access userspace in a preempt
> disabled region without disabling pagefaults and handle the resulting
> wreckage yourself.

I think I had missed that comment. My bad:-(

I will fix that with the code changes below. I will also double-check 
your comments again to see if I miss some others.

Cheers,
Longman


---------------------------[ cut here 
]-----------------------------------------

diff --git a/kernel/futex.c b/kernel/futex.c
index bc16eca..132a36d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3520,6 +3520,13 @@ static int futex_spin_on_owner(u32 __user *uaddr, 
u32 vpid,
        bool on_owner_pi_list = false;

        WRITE_ONCE(state->owner, current);
+retry:
+       /*
+        * The preempt_disable() has similar effect as pagefault_disable().
+        * As a result, we will have to disable page fault as well and 
handle
+        * the case of faulting in the futex word.
+        */
+       pagefault_disable();
        preempt_disable();
        for (;; loop--) {
                ret = futex_trylock(uaddr, vpid, &uval, true);
@@ -3648,6 +3655,14 @@ static int futex_spin_on_owner(u32 __user *uaddr, 
u32 vpid,
        }
 out:
        preempt_enable();
+       pagefault_enable();
+
+       if (ret == -EFAULT) {
+               ret = fault_in_user_writeable(uaddr);
+               if (!ret)
+                       goto retry;
+       }
+
        if (owner_task) {
                if (on_owner_pi_list)
                        task_pi_list_del(owner_task, state, false);