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:
-

[ message continues ]

i think that's quite right. I'm wondering why this never came up before? 

'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
-

[ message continues ]

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


-

[ message continues ]

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
-

[ message continues ]

But it has been modified by schedule_timeout?

-

[ message continues ]

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


-

[ message continues ]

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.

-

[ message continues ]

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

	tglx


-

[ message continues ]