Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 25, 2013 at 05:23:51PM +0100, Thomas Gleixner wrote:
> On Sat, 23 Nov 2013, Linus Torvalds wrote:
> 
> > On Sat, Nov 23, 2013 at 5:16 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > Now the question is why we queue the waiter _AFTER_ reading the user
> > > space value. The comment in the code is pretty non sensical:
> > >
> > >    * On the other hand, we insert q and release the hash-bucket only
> > >    * after testing *uaddr.  This guarantees that futex_wait() will NOT
> > >    * absorb a wakeup if *uaddr does not match the desired values
> > >    * while the syscall executes.
> > >
> > > There is no reason why we cannot queue _BEFORE_ reading the user space
> > > value. We just have to dequeue in all the error handling cases, but
> > > for the fast path it does not matter at all.
> > >
> > > CPU 0                                   CPU 1
> > >
> > >     val = *futex;
> > >     futex_wait(futex, val);
> > >
> > >     spin_lock(&hb->lock);
> > >
> > >     plist_add(hb, self);
> > >     smp_wmb();
> > >
> > >     uval = *futex;
> > >                                         *futex = newval;
> > >                                         futex_wake();
> > >
> > >                                         smp_rmb();
> > >                                         if (plist_empty(hb))
> > >                                            return;
> > > ...
> > 
> > This would seem to be a nicer approach indeed, without needing the
> > extra atomics.
> 
> I went through the issue with Peter and he noticed, that we need
> smp_mb() in both places. That's what we have right now with the
> spin_lock() and it is required as we need to guarantee that
> 
>  The waiter observes the change to the uaddr value after it added
>  itself to the plist
> 
>  The waker observes plist not empty if the change to uaddr was made
>  after the waiter checked the value.
> 
> 
> 	write(plist)		|	write(futex_uaddr)
> 	mb()			|	mb()
> 	read(futex_uaddr)	|	read(plist)
> 
> The spin_lock mb() on the waiter side does not help here because it
> happpens before the write(plist) and not after it.

Ah, note that spin_lock() is only a smp_mb() on x86, in general its an
ACQUIRE barrier which is weaker than a full mb and will not suffice in
this case even it if were in the right place.