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

[Thomas Gleixner]

On Sat, 23 Nov 2013, Linus Torvalds wrote:

> On Sat, Nov 23, 2013 at 5:16 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

Thanks,

	tglx
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