Re: [RFC][PATCH] vmscan: balance local_irq_disable() and local_irq_enable()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Wed, Feb 3, 2010 at 9:09 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > t On Wed, 2010-02-03 at 20:53 +0100, John Kacur wrote:
> >> Balance local_irq_disable() and local_irq_enable() as well as
> >> spin_lock_irq() and spin_lock_unlock_irq
> >>
> >> Signed-off-by: John Kacur <jkacur@redhat.com>
> >> ---
> >>  mm/vmscan.c |    3 ++-
> >>  1 files changed, 2 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index c26986c..b895025 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -1200,8 +1200,9 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
> >>               if (current_is_kswapd())
> >>                       __count_vm_events(KSWAPD_STEAL, nr_freed);
> >>               __count_zone_vm_events(PGSTEAL, zone, nr_freed);
> >> +             local_irq_enable();
> >>
> >> -             spin_lock(&zone->lru_lock);
> >> +             spin_lock_irq(&zone->lru_lock);
> >>               /*
> >>                * Put back any unfreeable pages.
> >>                */
> >
> >
> > The above looks wrong. I don't know the code, but just by looking at
> > where the locking and interrupts are, I can take a guess.
> >
> > Lets add a little more of the code:
> >
> >                local_irq_disable();
> >                if (current_is_kswapd())
> >                        __count_vm_events(KSWAPD_STEAL, nr_freed);
> >                __count_zone_vm_events(PGSTEAL, zone, nr_freed);
> >
> >                spin_lock(&zone->lru_lock);
> >                /*
> >
> > I'm guessing the __count_zone_vm_events and friends need interrupts
> > disabled here, probably due to per cpu stuff. But if you enable
> > interrupts before the spin_lock() you may let an interrupt come in and
> > invalidate what was done above it.
> >
> > So no, I do not think enabling interrupts here is a good thing.
> >
> 
> okay, and since we have already done local_irq_disable(), then that is
> why we only need the spin_lock() and not the spin_lock_irq() flavour?

Yes, spin_lock_irq() is equivalent to spin_lock() + irq_disable().
Now, we already disabled irq. then, we only need spin_lock().

So, I don't think shrink_inactive_list need any fix.


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