linux-kernel - Re: [PATCH] ARM: BUG() dies silently

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Date:	Thu, 31 Mar 2011 18:47:47 -0700
From:	Simon Glass <sjg@...omium.org>
To:	Omar Ramirez Luna <omar.ramirez@...com>
Cc:	Russell King <linux@....linux.org.uk>,
	lak <linux-arm-kernel@...ts.infradead.org>,
	lk <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: BUG() dies silently

You might try the generic bug patch and see if it also fixes this
problem - Simon

On Thu, Mar 31, 2011 at 1:15 PM, Omar Ramirez Luna <omar.ramirez@...com> wrote:
> There are some cases where the code generated for BUG() results
> into an infinite while loop without causing a null dereference,
> this ends on a kernel being stuck on a loop and the user without
> a clue of what happened.
>
> E.g.: lib/scatterlist.c : __sg_alloc_table
>
>        BUG_ON(nents > max_ents);
>  438:   9a000000        bls     440 <__sg_alloc_table+0x20>
>  43c:   eafffffe        b       43c <__sg_alloc_table+0x1c>
>
> Adding volatile makes the compiler to avoid optimizations on this
> code, which makes the panic to occur:
>
>        BUG_ON(nents > max_ents);
>  438:   9a000002        bls     448 <__sg_alloc_table+0x28>
>  43c:   e3a03000        mov     r3, #0
>  440:   e5833000        str     r3, [r3]
>  444:   eafffffc        b       43c <__sg_alloc_table+0x1c>
>
> Seen with gnu/linux cs arm-2010q1-202 and arm2010.09-50.
>
> Signed-off-by: Omar Ramirez Luna <omar.ramirez@...com>
> ---
>
> If needed I can change:
> ./arch/arm/mach-clps711x/include/mach/io.h:
>        #define __raw_readsb(p,d,l)  do { *(int *)0 = 0; } while (0)
> ./arch/arm/mach-clps711x/include/mach/io.h:
>        #define __raw_readsl(p,d,l)  do { *(int *)0 = 0; } while (0)
> ./arch/arm/mach-clps711x/include/mach/io.h:
>        #define __raw_writesb(p,d,l) do { *(int *)0 = 0; } while (0)
> ./arch/arm/mach-clps711x/include/mach/io.h:
>        #define __raw_writesl(p,d,l) do { *(int *)0 = 0; } while (0)
> ./arch/arm/kernel/traps.c:      *(int *)0 = 0;
>
>
>  arch/arm/include/asm/bug.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/arch/arm/include/asm/bug.h b/arch/arm/include/asm/bug.h
> index 4d88425..a58d863 100644
> --- a/arch/arm/include/asm/bug.h
> +++ b/arch/arm/include/asm/bug.h
> @@ -12,7 +12,7 @@ extern void __bug(const char *file, int line) __attribute__((noreturn));
>  #else
>
>  /* this just causes an oops */
> -#define BUG()          do { *(int *)0 = 0; } while (1)
> +#define BUG()          do { *(volatile int *)0 = 0; } while(1)
>
>  #endif
>
> --
> 1.7.1
>
> --
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