linux-kernel - Re: [RFC][PATCH] vmscan: balance local_irq_disable() and local_irq

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Date:	Fri, 5 Feb 2010 17:05:24 +0100
From:	John Kacur <jkacur@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	rostedt@...dmis.org, lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] vmscan: balance local_irq_disable() and 
	local_irq_enable()

On Thu, Feb 4, 2010 at 1:22 AM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> On Wed, Feb 3, 2010 at 9:09 PM, Steven Rostedt <rostedt@...dmis.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@...hat.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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