Re: Memory barrier needed with wake_up_process()?

[Paul E. McKenney]

On Fri, Sep 02, 2016 at 04:29:19PM -0400, Alan Stern wrote:
> On Fri, 2 Sep 2016, Paul E. McKenney wrote:
> 
> > On Fri, Sep 02, 2016 at 02:10:13PM -0400, Alan Stern wrote:
> > > Paul, Peter, and Ingo:
> > > 
> > > This must have come up before, but I don't know what was decided.
> > > 
> > > Isn't it often true that a memory barrier is needed before a call to 
> > > wake_up_process()?  A typical scenario might look like this:
> > > 
> > > 	CPU 0
> > > 	-----
> > > 	for (;;) {
> > > 		set_current_state(TASK_INTERRUPTIBLE);
> > > 		if (signal_pending(current))
> > > 			break;
> > > 		if (wakeup_flag)
> > > 			break;
> > > 		schedule();
> > > 	}
> > > 	__set_current_state(TASK_RUNNING);
> > > 	wakeup_flag = 0;
> > > 
> > > 
> > > 	CPU 1
> > > 	-----
> > > 	wakeup_flag = 1;
> > > 	wake_up_process(my_task);
> > > 
> > > The underlying pattern is:
> > > 
> > > 	CPU 0				CPU 1
> > > 	-----				-----
> > > 	write current->state		write wakeup_flag
> > > 	smp_mb();
> > > 	read wakeup_flag		read my_task->state
> > > 
> > > where set_current_state() does the write to current->state and 
> > > automatically adds the smp_mb(), and wake_up_process() reads 
> > > my_task->state to see whether the task needs to be woken up.
> > > 
> > > The kerneldoc for wake_up_process() says that it has no implied memory
> > > barrier if it doesn't actually wake anything up.  And even when it
> > > does, the implied barrier is only smp_wmb, not smp_mb.
> > > 
> > > This is the so-called SB (Store Buffer) pattern, which is well known to
> > > require a full smp_mb on both sides.  Since wake_up_process() doesn't
> > > include smp_mb(), isn't it correct that the caller must add it
> > > explicitly?
> > > 
> > > In other words, shouldn't the code for CPU 1 really be:
> > > 
> > > 	wakeup_flag = 1;
> > > 	smp_mb();
> > > 	wake_up_process(task);
> > > 
> > > If my reasoning is correct, then why doesn't wake_up_process() include 
> > > this memory barrier automatically, the way set_current_state() does?  
> > > There could be an alternate version (__wake_up_process()) which omits 
> > > the barrier, just like __set_current_state().
> > 
> > A common case uses locking, in which case additional memory barriers
> > inside of the wait/wakeup functions are not needed.  Any accesses made
> > while holding the lock before invoking the wakeup function (e.g.,
> > wake_up()) are guaranteed to be seen after acquiring that same
> > lock following return from the wait function (e.g., wait_event()).
> > In this case, adding barriers to the wait and wakeup functions would
> > just add overhead.
> > 
> > But yes, this decision does mean that people using the wait/wakeup
> > functions without locking need to be more careful.  Something like
> > this:
> > 
> > 	/* prior accesses. */
> > 	smp_mb();
> > 	wakeup_flag = 1;
> > 	wake_up(...);
> > 
> > And on the other task:
> > 
> > 	wait_event(... wakeup_flag == 1 ...);
> > 	smp_mb();
> > 	/* The waker's prior accesses will be visible here. */
> > 
> > Or am I missing your point?
> 
> I'm afraid so.  The code doesn't use wait_event(), in part because
> there's no wait_queue (since only one task is involved).

Ah, got it.

The required pattern should be very similar, however.

> But maybe there's another barrier which needs to be fixed.  Felipe, can
> you check to see if received_cbw() is getting called in
> get_next_command(), and if so, what value it returns?  Or is the
> preceding sleep_thread() the one that never wakes up?
> 
> It could be that the smp_wmb() in wakeup_thread() needs to be smp_mb().  
> The reason being that get_next_command() runs outside the protection of 
> the spinlock.

This sounds very likely to me.

							Thanx, Paul