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

[Steven Rostedt]

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 <jka...@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.

-- Steve

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[John Kacur]

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?

[KOSAKI Motohiro]

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.

[John Kacur]

Thanks for the explanation!