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

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 25 Nov 2013, Darren Hart wrote:
> On Mon, 2013-11-25 at 20:47 +0100, Thomas Gleixner wrote:
> > > which restores the ordering guarantee, which the hash bucket lock
> > > provided so far.
> > 
> > Actually that's not true by design, it just happens to work.
> > 
> > atomic_inc() on x86 is a "lock incl".
> > 
> >  The LOCK prefix just guarantees that the cache line which is affected
> >  by the INCL is locked. And it guarantees that locked operations
> >  serialize all outstanding load and store operations.
> > 
> >  But Documentation/atomic_ops.txt says about atomic_inc():
> > 
> >  "One very important aspect of these two routines is that they DO NOT
> >   require any explicit memory barriers.  They need only perform the
> >   atomic_t counter update in an SMP safe manner."
> > 
> >  So while this has a barrier on x86, it's not guaranteed.
> 
> 
> But it is guaranteed to be "in an SMP safe manner"... which I guess just
> means that two writes will not intermix bytes, but no guarantee that the
> value will be visible to other CPUs unless some kind of barrier is
> explicitly imposed.
> 
> Correct?

Yep, that's what it sayes.
 
> > So now your code melts down to:
> > 
> >  write(hb->waiters)    |    write(uaddr)
> >  mb                    |    read(hb->waiters)
> >  read(uaddr)
> > 
> > I fear you simply managed to make the window small enough that your
> > testing was not longer able expose it.
> 
> Does seem to be the case.

You must be aware, that between the the write(uaddr) and the
read(hb->waiters) is the syscall, i.e. a user/kernel space
transition.

sysenter/syscall have no documented barriers inside, but we don't know
whether the actual hw implementation provides one or if the memory ops
between modifying uaddr and reaching the read(hb->waiters) point
including the real atomic op on the waiter side are good enough to
paper over the issue.

Thanks,

	tglx