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

[John Kacur <jkacur@redhat.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?

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