On Fri, 21 Feb 2014, Liu, Chuansheng wrote:
> > > > I think you have a point there, but not on x86 wherre the atomic_dec
> > > > and the spinlock on the queueing side are full barriers. For non-x86
> > > > there is definitely a potential issue.
> > > >
> > > But even on X86, spin_unlock has no full barrier, the following scenario:
> > > CPU0 CPU1
> > > spin_lock
> > > atomic_dec_and_test
> > > insert into queue
> > > spin_unlock
> > > checking waitqueue_active
> >
> > But CPU0 sees the 0, right?
> Not be clear here:)
> The atomic_read has no barrier.
>
> Found commit 6cb2a21049b89 has one similar smp_mb() calling before
> waitqueue_active() on one X86 CPU.
Indeed, you are completely right. Great detective work!
I'm inclined to remove the waitqueue_active() alltogether. It's
creating more headache than it's worth.
Thanks,
tglx
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