Re: BUG(): sched.c: Line 944 - 2.5.35

Re: BUG(): sched.c: Line 944 - 2.5.35

[Adam J. Richter]

Shawn Starr wrote:

>Kernel 2.5.35:
>
>code resides in main schedule() function:
>
>if (unlikely(in_atomic()))
>   BUG();


	That line prevously checked in_interrupt (in 2.5.34) instead of
in_atomic.  If you have CONFIG_PREEMPT defined, the definition of in_atomic
in linux-2.5.35/include/asm-i386/hardirq.h is:

# define in_atomic()    (preempt_count() != kernel_locked())

	When I see this problem at boot, preempt_count() returns 0x4000000
(PREEMPT_ACTIVE) and kernel_locked() returns 0.

	I don't understand the semantics of PREEMPT_ACTIVE to know
whether to

	(1) change the test back to using in_interrupt instead of in_atomic, or
	(2) change the definition of in_atomic(), or
	(3) look for a bug somewhere else.

	However, I know experimentally that changing the test back to
using in_interrupt() results in a possibly unrelated BUG() at line 279
of rmap.c:

void page_remove_rmap(struct page * page, pte_t * ptep)
{
        pte_addr_t pte_paddr = ptep_to_paddr(ptep);
        struct pte_chain *pc;

        if (!page || !ptep)
                BUG();
        if (!pfn_valid(page_to_pfn(page)) || PageReserved(page))
                return;

        pte_chain_lock(page);

        BUG_ON(page->pte.direct == 0);



Adam J. Richter     __     ______________   575 Oroville Road
adam@yggdrasil.com     \ /                  Milpitas, California 95035
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                         "Free Software For The Rest Of Us."

Re: BUG(): sched.c: Line 944 - 2.5.35

[Robert Love]

On Mon, 2002-09-16 at 12:36, Adam J. Richter wrote:

> 	When I see this problem at boot, preempt_count() returns 0x4000000
> (PREEMPT_ACTIVE) and kernel_locked() returns 0.
> 
> 	I don't understand the semantics of PREEMPT_ACTIVE to know
> whether to
> 
> 	(1) change the test back to using in_interrupt instead of in_atomic, or
> 	(2) change the definition of in_atomic(), or
> 	(3) look for a bug somewhere else.

There are two problems: First, PREEMPT_ACTIVE is indeed set on entry to
schedule() from preempt_schedule() so we need to check for that too. 
Second, the BUG() is catching a bit of issues... you want something
like:

-	if (unlikely(in_atomic()))
-		BUG();
+	if (unlikely(in_atomic() && preempt_count() != PREEMPT_ACTIVE)) {
+		printk(KERN_ERR "schedule() called while non-atomic!\n");
+		show_stack(NULL);
+	}

I will send a patch to Linus.

> 	However, I know experimentally that changing the test back to
> using in_interrupt() results in a possibly unrelated BUG() at line 279
> of rmap.c:

This is unrelated.

	Robert Love

Re: BUG(): sched.c: Line 944 - 2.5.35

[venom]

OK, I will try this tomorrow morning

Luigi

On 16 Sep 2002, Robert Love wrote:

> Date: 16 Sep 2002 14:42:13 -0400
> From: Robert Love <rml@tech9.net>
> To: Adam J. Richter <adam@yggdrasil.com>
> Cc: linux-kernel@vger.kernel.org, spstarr@sh0n.net, venom@sns.it
> Subject: Re: BUG(): sched.c: Line 944 - 2.5.35
>
> On Mon, 2002-09-16 at 12:36, Adam J. Richter wrote:
>
> > 	When I see this problem at boot, preempt_count() returns 0x4000000
> > (PREEMPT_ACTIVE) and kernel_locked() returns 0.
> >
> > 	I don't understand the semantics of PREEMPT_ACTIVE to know
> > whether to
> >
> > 	(1) change the test back to using in_interrupt instead of in_atomic, or
> > 	(2) change the definition of in_atomic(), or
> > 	(3) look for a bug somewhere else.
>
> There are two problems: First, PREEMPT_ACTIVE is indeed set on entry to
> schedule() from preempt_schedule() so we need to check for that too.
> Second, the BUG() is catching a bit of issues... you want something
> like:
>
> -	if (unlikely(in_atomic()))
> -		BUG();
> +	if (unlikely(in_atomic() && preempt_count() != PREEMPT_ACTIVE)) {
> +		printk(KERN_ERR "schedule() called while non-atomic!\n");
> +		show_stack(NULL);
> +	}
>
> I will send a patch to Linus.
>
> > 	However, I know experimentally that changing the test back to
> > using in_interrupt() results in a possibly unrelated BUG() at line 279
> > of rmap.c:
>
> This is unrelated.
>
> 	Robert Love
>
> -
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BUG(): sched.c: Line 944 - 2.5.35

[Shawn Starr]

Machine: Athlon MP 2000+ 512MB DDR Registered
Motherboard: A7M266-D

Kernel 2.5.35:

code resides in main schedule() function:

if (unlikely(in_atomic()))
   BUG();

:(

--
Shawn Starr, sh0n.net, <spstarr@sh0n.net>
Maintainer: -shawn kernel patches: http://xfs.sh0n.net/2.4/