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

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Message-ID: <57F05EE9.20702@hpe.com>
Date:   Sat, 1 Oct 2016 21:12:09 -0400
From:   Waiman Long <waiman.long@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Jason Low <jason.low2@....com>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v3 07/13] futex: Throughput-optimized (TP) futexes

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);