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    SubjectRe: [RFC][PATCH] vmscan: balance local_irq_disable() and local_irq_enable()
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    On Thu, Feb 4, 2010 at 1:22 AM, KOSAKI Motohiro
    <kosaki.motohiro@jp.fujitsu.com> wrote:
    >> 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.
    >

    Thanks for the explanation!
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