Re: [PATCH][5/8] Arch agnostic completely out of line locks / ppc64

[Paul Mackerras <paulus@samba.org>]

Zwane,

Just got a chance to look at the new out-of-line spinlock stuff
(better late than never :).  I see a couple of problems there.  First,
we now go two levels deep on SMP && PREEMPT: spin_lock is _spin_lock,
which is out of line in kernel/sched.c.  That calls
__preempt_spin_lock, which is out of line in kernel/sched.c, and isn't
in the .text.lock section.  So if we get a timer interrupt in there,
we won't attribute the profile tick to the original caller.

The second problem is that __preempt_spin_lock doesn't do the yield to
the hypervisor which we need to do on shared processor systems.  This
is actually a long-standing problem, not one you have just introduced,
but I have only just noticed it.  I can't make cpu_relax do the yield
because the yield is a directed yield to a specific other virtual cpu
(it says "give the rest of my timeslice to that guy over there") and I
need the value in the lock variable in order to know who is holding
the lock.

Regards,
Paul.