Mailing List Archive: [PATCH] adapt page_lock_anon_vma() to PREEMPT

[PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU

Feb 25, 2007, 12:06 PM

Post #1 of 5 (442 views)

page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
theoretically possible that slab returns anon_vma's memory to the system
before we do spin_unlock(&anon_vma->lock).

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- WQ/mm/rmap.c~ 2007-02-18 22:56:49.000000000 +0300
+++ WQ/mm/rmap.c 2007-02-25 22:43:00.000000000 +0300
@@ -183,7 +183,7 @@ void __init anon_vma_init(void)
*/
static struct anon_vma *page_lock_anon_vma(struct page *page)
{
- struct anon_vma *anon_vma = NULL;
+ struct anon_vma *anon_vma;
unsigned long anon_mapping;

rcu_read_lock();
@@ -195,9 +195,16 @@ static struct anon_vma *page_lock_anon_v

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
spin_lock(&anon_vma->lock);
+ return anon_vma;
out:
rcu_read_unlock();
- return anon_vma;
+ return NULL;
+}
+
+static void page_unlock_anon_vma(struct anon_vma *anon_vma)
+{
+ spin_unlock(&anon_vma->lock);
+ rcu_read_unlock();
}

/*
@@ -333,7 +340,8 @@ static int page_referenced_anon(struct p
if (!mapcount)
break;
}
- spin_unlock(&anon_vma->lock);
+
+ page_unlock_anon_vma(anon_vma);
return referenced;
}

@@ -809,7 +817,8 @@ static int try_to_unmap_anon(struct page
!page_mapped(page))
break;
}
- spin_unlock(&anon_vma->lock);
+
+ page_unlock_anon_vma(anon_vma);
return ret;
}

-
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Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU [ In reply to ]

akpm at linux-foundation

Feb 27, 2007, 12:25 PM

Post #2 of 5 (415 views)

Permalink

> On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@tv-sign.ru> wrote:
> page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> theoretically possible that slab returns anon_vma's memory to the system
> before we do spin_unlock(&anon_vma->lock).
>
> ...
>
> +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> +{
> + spin_unlock(&anon_vma->lock);
> + rcu_read_unlock();
> }

It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.

Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a
no-op if !PREEMPT_RCU.

-
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Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU [ In reply to ]

oleg at tv-sign

Feb 27, 2007, 1:46 PM

Post #3 of 5 (422 views)

Permalink

On 02/27, Andrew Morton wrote:
>
> > On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@tv-sign.ru> wrote:
> > page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> > PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> > theoretically possible that slab returns anon_vma's memory to the system
> > before we do spin_unlock(&anon_vma->lock).
> >
> > ...
> >
> > +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> > +{
> > + spin_unlock(&anon_vma->lock);
> > + rcu_read_unlock();
> > }
>
> It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.

Actually, we don't in this case. This patch in essence moves "preempt_enable"
from "lock" to "unlock" side. Zero impact for non-PREEMPT_RCU builds, except
.text grows a bit.

Before this patch, page_lock_anon_vma() does preempt_enable() before return,
but this can't help because ->preempt_count was incremented by spin_lock().

> Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a
> no-op if !PREEMPT_RCU.

I also thought about things like

rcu_read_lock_when_we_know_that_preemption_disabled()
rcu_read_lock_when_we_know_that_irqs_disabled()

which are noops when !PREEMPT_RCU.

Oleg.

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Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU [ In reply to ]

paulmck at linux

Feb 27, 2007, 3:13 PM

Post #4 of 5 (434 views)

Permalink

On Tue, Feb 27, 2007 at 12:25:17PM -0800, Andrew Morton wrote:
> > On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@tv-sign.ru> wrote:
> > page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> > PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> > theoretically possible that slab returns anon_vma's memory to the system
> > before we do spin_unlock(&anon_vma->lock).
> >
> > ...
> >
> > +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> > +{
> > + spin_unlock(&anon_vma->lock);
> > + rcu_read_unlock();
> > }
>
> It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.
>
> Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a
> no-op if !PREEMPT_RCU.

We were doing double preempt_disable() before as well. The only
difference is that we moved RCU preempt_enable() (it used to be inside the
critical section, and now it is after the corresponding spin_unlock()).

I hope to keep RCU API proliferation down to a dull roar, but if we
need more APIs, we need more APIs. This example does not demonstrate
that need to me, however.

Thanx, Paul
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Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU [ In reply to ]

hugh at veritas

Feb 28, 2007, 11:48 AM

Post #5 of 5 (417 views)

Permalink

On Sun, 25 Feb 2007, Oleg Nesterov wrote:

> page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> theoretically possible that slab returns anon_vma's memory to the system
> before we do spin_unlock(&anon_vma->lock).
>
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Acked-by: Hugh Dickins <hugh@veritas.com>

Thanks for doing this, and sorry for my delay.

Hugh

>
> --- WQ/mm/rmap.c~ 2007-02-18 22:56:49.000000000 +0300
> +++ WQ/mm/rmap.c 2007-02-25 22:43:00.000000000 +0300
> @@ -183,7 +183,7 @@ void __init anon_vma_init(void)
> */
> static struct anon_vma *page_lock_anon_vma(struct page *page)
> {
> - struct anon_vma *anon_vma = NULL;
> + struct anon_vma *anon_vma;
> unsigned long anon_mapping;
>
> rcu_read_lock();
> @@ -195,9 +195,16 @@ static struct anon_vma *page_lock_anon_v
>
> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> spin_lock(&anon_vma->lock);
> + return anon_vma;
> out:
> rcu_read_unlock();
> - return anon_vma;
> + return NULL;
> +}
> +
> +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> +{
> + spin_unlock(&anon_vma->lock);
> + rcu_read_unlock();
> }
>
> /*
> @@ -333,7 +340,8 @@ static int page_referenced_anon(struct p
> if (!mapcount)
> break;
> }
> - spin_unlock(&anon_vma->lock);
> +
> + page_unlock_anon_vma(anon_vma);
> return referenced;
> }
>
> @@ -809,7 +817,8 @@ static int try_to_unmap_anon(struct page
> !page_mapped(page))
> break;
> }
> - spin_unlock(&anon_vma->lock);
> +
> + page_unlock_anon_vma(anon_vma);
> return ret;
> }
>
>
-
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