[rfc][patch] futex: restartable futex_wait?

[rfc][patch] futex: restartable futex_wait?

[Nick Piggin]

Hi Ingo,

I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically,
it tests whether the SA_RESTART flag works for the sem_wait operation.

I see sem_wait is implemented with futex_wait, so I wonder whether we
can make it restartable? Am I going about it the right way? (Seems to
fix the testcase here).

Thanks,
Nick

--
Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -978,6 +978,7 @@ static void unqueue_me_pi(struct futex_q
 	drop_futex_key_refs(&q->key);
 }
 
+static long futex_wait_restart(struct restart_block *restart);
 static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time)
 {
 	struct task_struct *curr = current;
@@ -1077,11 +1078,22 @@ static int futex_wait(u32 __user *uaddr,
 		return 0;
 	if (time == 0)
 		return -ETIMEDOUT;
+
 	/*
 	 * We expect signal_pending(current), but another thread may
 	 * have handled it for us already.
 	 */
-	return -EINTR;
+	if (time == MAX_SCHEDULE_TIMEOUT)
+		return -ERESTARTSYS;
+	else {
+		struct restart_block *restart;
+		restart = &current_thread_info()->restart_block;
+		restart->fn = futex_wait_restart;
+		restart->arg0 = (unsigned long)uaddr;
+		restart->arg1 = (unsigned long)val;
+		restart->arg2 = time;
+		return -ERESTART_RESTARTBLOCK;
+	}
 
  out_unlock_release_sem:
 	queue_unlock(&q, hb);
@@ -1091,6 +1103,17 @@ static int futex_wait(u32 __user *uaddr,
 	return ret;
 }
 
+static long futex_wait_restart(struct restart_block *restart)
+{
+	u32 __user *uaddr = (u32 __user *)restart->arg0;
+	u32 val = (u32)restart->arg1;
+	unsigned long time = restart->arg2;
+
+	restart->fn = do_no_restart_syscall;
+	return (long)futex_wait(uaddr, val, time);
+}
+
+
 /*
  * Userspace tried a 0 -> TID atomic transition of the futex value
  * and failed. The kernel side here does the whole locking operation:

Re: [rfc][patch] futex: restartable futex_wait?

[Ingo Molnar]

* Nick Piggin <npiggin@suse.de> wrote:

> Hi Ingo,
> 
> I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically, 
> it tests whether the SA_RESTART flag works for the sem_wait operation.
> 
> I see sem_wait is implemented with futex_wait, so I wonder whether we 
> can make it restartable? Am I going about it the right way? (Seems to 
> fix the testcase here).

i think that's quite right. I'm wondering why this never came up before? 
But your fix is not complete i think:

> +             restart->arg2 = time;
> +             return -ERESTART_RESTARTBLOCK;
> +     }

'time' here is relative, so the restarted syscall will do a /full/ wait 
again.

maybe we should rather convert futex timed-waits to hrtimers? Thomas?

	Ingo

Re: [rfc][patch] futex: restartable futex_wait?

[Thomas Gleixner]

On Thu, 2007-03-08 at 18:29 +0100, Ingo Molnar wrote:
> * Nick Piggin <npiggin@suse.de> wrote:
> 
> > Hi Ingo,
> > 
> > I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically, 
> > it tests whether the SA_RESTART flag works for the sem_wait operation.

Not sure, whether the testcase is correct or not. See below

> > I see sem_wait is implemented with futex_wait, so I wonder whether we 
> > can make it restartable? Am I going about it the right way? (Seems to 
> > fix the testcase here).
> 
> i think that's quite right. I'm wondering why this never came up before? 
> But your fix is not complete i think:
> 
> > +             restart->arg2 = time;
> > +             return -ERESTART_RESTARTBLOCK;
> > +     }
> 
> 'time' here is relative, so the restarted syscall will do a /full/ wait 
> again.
> 
> maybe we should rather convert futex timed-waits to hrtimers? Thomas?

The problem is that the original API is based on relative time and
therefor can not be changed. 

sem_wait returns -EINTR to the application when it is interrupted, while
pthread_mutex_lock does not.

http://www.opengroup.org/onlinepubs/009695399/functions/sem_wait.html

http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html

We need to create a seperate op for the futex - just like the pi_futex
and use absolute time there too. 

	tglx

Re: [rfc][patch] futex: restartable futex_wait?

[Nick Piggin]

On Fri, Mar 09, 2007 at 12:02:31AM +0100, Thomas Gleixner wrote:
> On Thu, 2007-03-08 at 18:29 +0100, Ingo Molnar wrote:
> > * Nick Piggin <npiggin@suse.de> wrote:
> > 
> > > Hi Ingo,
> > > 
> > > I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically, 
> > > it tests whether the SA_RESTART flag works for the sem_wait operation.
> 
> Not sure, whether the testcase is correct or not. See below
> 
> > > I see sem_wait is implemented with futex_wait, so I wonder whether we 
> > > can make it restartable? Am I going about it the right way? (Seems to 
> > > fix the testcase here).
> > 
> > i think that's quite right. I'm wondering why this never came up before? 
> > But your fix is not complete i think:
> > 
> > > +             restart->arg2 = time;
> > > +             return -ERESTART_RESTARTBLOCK;
> > > +     }
> > 
> > 'time' here is relative, so the restarted syscall will do a /full/ wait 
> > again.
> > 
> > maybe we should rather convert futex timed-waits to hrtimers? Thomas?
> 
> The problem is that the original API is based on relative time and
> therefor can not be changed. 
> 
> sem_wait returns -EINTR to the application when it is interrupted, while
> pthread_mutex_lock does not.

But this still means sem_wait should restart if SA_RESTART is set, right?

And pthread_mutex_lock could be implemented to not return -EINTR, even if
futex_wait does, couldn't it? (I guess it probably already is, considering
that futex_wait alsready returns -EINTR).

> 
> http://www.opengroup.org/onlinepubs/009695399/functions/sem_wait.html
> 
> http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html
> 
> We need to create a seperate op for the futex - just like the pi_futex
> and use absolute time there too. 
> 
> 	tglx
> 

Re: [rfc][patch] futex: restartable futex_wait?

[Nick Piggin]

On Thu, Mar 08, 2007 at 06:29:02PM +0100, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@suse.de> wrote:
> 
> > Hi Ingo,
> > 
> > I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically, 
> > it tests whether the SA_RESTART flag works for the sem_wait operation.
> > 
> > I see sem_wait is implemented with futex_wait, so I wonder whether we 
> > can make it restartable? Am I going about it the right way? (Seems to 
> > fix the testcase here).
> 
> i think that's quite right. I'm wondering why this never came up before? 
> But your fix is not complete i think:
> 
> > +             restart->arg2 = time;
> > +             return -ERESTART_RESTARTBLOCK;
> > +     }
> 
> 'time' here is relative, so the restarted syscall will do a /full/ wait 
> again.

But it has been modified by schedule_timeout?

Re: [rfc][patch] futex: restartable futex_wait?

[Thomas Gleixner]

On Fri, 2007-03-09 at 06:10 +0100, Nick Piggin wrote:
> > i think that's quite right. I'm wondering why this never came up before? 
> > But your fix is not complete i think:
> > 
> > > +             restart->arg2 = time;
> > > +             return -ERESTART_RESTARTBLOCK;
> > > +     }
> > 
> > 'time' here is relative, so the restarted syscall will do a /full/ wait 
> > again.
> 
> But it has been modified by schedule_timeout?

But this does not change the syscall registers, so it is restarted in
the same way. We need a new futex OP for this, which takes absolute time
like the PI futex op does.

	tglx

Re: [rfc][patch] futex: restartable futex_wait?

[Nick Piggin]

On Fri, Mar 09, 2007 at 10:38:35AM +0100, Thomas Gleixner wrote:
> On Fri, 2007-03-09 at 06:10 +0100, Nick Piggin wrote:
> > > i think that's quite right. I'm wondering why this never came up before? 
> > > But your fix is not complete i think:
> > > 
> > > > +             restart->arg2 = time;
> > > > +             return -ERESTART_RESTARTBLOCK;
> > > > +     }
> > > 
> > > 'time' here is relative, so the restarted syscall will do a /full/ wait 
> > > again.
> > 
> > But it has been modified by schedule_timeout?
> 
> But this does not change the syscall registers, so it is restarted in
> the same way. We need a new futex OP for this, which takes absolute time
> like the PI futex op does.

Forgive me if I'm missing something here, but I'm using the restart block
and saving the updated value of time in ->arg2, and using that as the new
time parameter passed into futex_wait from futex_wait_restart.

Re: [rfc][patch] futex: restartable futex_wait?

[Thomas Gleixner]

On Fri, 2007-03-09 at 13:24 +0100, Nick Piggin wrote:
> > > > 'time' here is relative, so the restarted syscall will do a /full/ wait 
> > > > again.
> > > 
> > > But it has been modified by schedule_timeout?
> > 
> > But this does not change the syscall registers, so it is restarted in
> > the same way. We need a new futex OP for this, which takes absolute time
> > like the PI futex op does.
> 
> Forgive me if I'm missing something here, but I'm using the restart block
> and saving the updated value of time in ->arg2, and using that as the new
> time parameter passed into futex_wait from futex_wait_restart.

Oops. I went into confusion mode. You are right, the restart block keeps
that.

	tglx