Patchwork [1/2] net: don't use in_atomic() in gfp_any()

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Submitter Andrew Morton
Date Feb. 11, 2009, 9:27 p.m.
Message ID <200902112127.n1BLRJeJ031575@imap1.linux-foundation.org>
Download mbox | patch
Permalink /patch/22941/
State Accepted
Delegated to: David Miller
Headers show

Comments

Andrew Morton - Feb. 11, 2009, 9:27 p.m.
From: Andrew Morton <akpm@linux-foundation.org>

The problem is that in_atomic() will return false inside spinlocks if
CONFIG_PREEMPT=n.  This will lead to deadlockable GFP_KERNEL allocations
from spinlocked regions.

Secondly, if CONFIG_PREEMPT=y, this bug solves itself because networking
will instead use GFP_ATOMIC from this callsite.  Hence we won't get the
might_sleep() debugging warnings which would have informed us of the buggy
callsites.

Solve both these problems by switching to in_interrupt().  Now, if someone
runs a gfp_any() allocation from inside spinlock we will get the warning
if CONFIG_PREEMPT=y.

I reviewed all callsites and most of them were too complex for my little
brain and none of them documented their interface requirements.  I have no
idea what this patch will do.

Cc: Andi Kleen <andi@firstfloor.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/net/sock.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
Tetsuo Handa - Feb. 11, 2009, 9:38 p.m.
Andrew Morton wrote:
> Solve both these problems by switching to in_interrupt().  Now, if someone
> runs a gfp_any() allocation from inside spinlock we will get the warning
> if CONFIG_PREEMPT=y.

>  static inline gfp_t gfp_any(void)
>  {
> -	return in_atomic() ? GFP_ATOMIC : GFP_KERNEL;
> +	return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
>  }
sed -i -e 's/in_softirq/in_interrupt/' ?
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Andrew Morton - Feb. 11, 2009, 9:48 p.m.
On Thu, 12 Feb 2009 06:38:55 +0900
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Andrew Morton wrote:
> > Solve both these problems by switching to in_interrupt().  Now, if someone
> > runs a gfp_any() allocation from inside spinlock we will get the warning
> > if CONFIG_PREEMPT=y.
> 
> >  static inline gfp_t gfp_any(void)
> >  {
> > -	return in_atomic() ? GFP_ATOMIC : GFP_KERNEL;
> > +	return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
> >  }
> sed -i -e 's/in_softirq/in_interrupt/' ?

heh, good question.

I did all my testing with in_softirq() - the changelog didn't catch up.



Is gfp_any() supposed to be usable from hardirq context?

If so, we should use in_interrupt().

If not, we should use in_softirq(), and we'll then get might_sleep()
warnings if anyone uses gfp_any() from hard irq context.




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Andrew Morton - Feb. 11, 2009, 9:49 p.m.
Resend with cc's restored.  Please DO NOT edit the Cc: line :(

On Thu, 12 Feb 2009 06:38:55 +0900
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Andrew Morton wrote:
> > Solve both these problems by switching to in_interrupt().  Now, if someone
> > runs a gfp_any() allocation from inside spinlock we will get the warning
> > if CONFIG_PREEMPT=y.
> 
> >  static inline gfp_t gfp_any(void)
> >  {
> > -	return in_atomic() ? GFP_ATOMIC : GFP_KERNEL;
> > +	return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
> >  }
> sed -i -e 's/in_softirq/in_interrupt/' ?

heh, good question.

I did all my testing with in_softirq() - the changelog didn't catch up.



Is gfp_any() supposed to be usable from hardirq context?

If so, we should use in_interrupt().

If not, we should use in_softirq(), and we'll then get might_sleep()
warnings if anyone uses gfp_any() from hard irq context.




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David Miller - Feb. 11, 2009, 10:02 p.m.
From: Andrew Morton <akpm@linux-foundation.org>
Date: Wed, 11 Feb 2009 13:48:42 -0800

> Is gfp_any() supposed to be usable from hardirq context?
> 
> If so, we should use in_interrupt().
> 
> If not, we should use in_softirq(), and we'll then get might_sleep()
> warnings if anyone uses gfp_any() from hard irq context.

It never will be used in hardware IRQ context, at least
in the networking.

And all uses in the tree are.. networking :-)
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David Miller - Feb. 13, 2009, 12:43 a.m.
From: akpm@linux-foundation.org
Date: Wed, 11 Feb 2009 13:27:19 -0800

> The problem is that in_atomic() will return false inside spinlocks if
> CONFIG_PREEMPT=n.  This will lead to deadlockable GFP_KERNEL allocations
> from spinlocked regions.
> 
> Secondly, if CONFIG_PREEMPT=y, this bug solves itself because networking
> will instead use GFP_ATOMIC from this callsite.  Hence we won't get the
> might_sleep() debugging warnings which would have informed us of the buggy
> callsites.
> 
> Solve both these problems by switching to in_interrupt().  Now, if someone
> runs a gfp_any() allocation from inside spinlock we will get the warning
> if CONFIG_PREEMPT=y.
> 
> I reviewed all callsites and most of them were too complex for my little
> brain and none of them documented their interface requirements.  I have no
> idea what this patch will do.
> 
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Applied.
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Patch

diff -puN include/net/sock.h~net-dont-use-in_atomic-in-gfp_any include/net/sock.h
--- a/include/net/sock.h~net-dont-use-in_atomic-in-gfp_any
+++ a/include/net/sock.h
@@ -1313,7 +1313,7 @@  static inline int sock_writeable(const s
 
 static inline gfp_t gfp_any(void)
 {
-	return in_atomic() ? GFP_ATOMIC : GFP_KERNEL;
+	return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
 }
 
 static inline long sock_rcvtimeo(const struct sock *sk, int noblock)