linux-kernel - Re: [PATCH] slub: Disable the lockless allocator

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Date:	Mon, 28 Mar 2011 09:12:24 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <cl@...ux.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	akpm@...ux-foundation.org, tj@...nel.org, npiggin@...nel.dk,
	rientjes@...gle.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, tj@...nel.org
Subject: Re: [PATCH] slub: Disable the lockless allocator

On 3/27/11 4:57 AM, Christoph Lameter wrote:
> But then the same fix must also be used in the asm code or the fallback
> (turns out that the fallback is always used in kmem_cache_init since
> the instruction patching comes later).
>
> Patch boots fine both in UP and SMP mode
>
>
>
>
> Subject: percpu: Omit segment prefix in the UP case for cmpxchg_double
>
> Omit the segment prefix in the UP case. GS is not used then
> and we will generate segfaults if cmpxchg16b is used otherwise.
>
> Signed-off-by: Linus Torvalds<torvalds@...ux-foundation.org>
> Signed-off-by: Christoph Lameter<cl@...ux.com>
>
>   arch/x86/include/asm/percpu.h |   10 ++++++----
>   1 files changed, 6 insertions(+), 4 deletions(-)
>
> Index: linux-2.6/arch/x86/include/asm/percpu.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/percpu.h	2011-03-26 20:43:03.994089001 -0500
> +++ linux-2.6/arch/x86/include/asm/percpu.h	2011-03-26 20:43:22.414089004 -0500
> @@ -45,7 +45,7 @@
>   #include<linux/stringify.h>
>
>   #ifdef CONFIG_SMP
> -#define __percpu_arg(x)		"%%"__stringify(__percpu_seg)":%P" #x
> +#define __percpu_prefix		"%%"__stringify(__percpu_seg)":"
>   #define __my_cpu_offset		percpu_read(this_cpu_off)
>
>   /*
> @@ -62,9 +62,11 @@
>   	(typeof(*(ptr)) __kernel __force *)tcp_ptr__;	\
>   })
>   #else
> -#define __percpu_arg(x)		"%P" #x
> +#define __percpu_prefix		""
>   #endif
>
> +#define __percpu_arg(x)		__percpu_prefix "%P" #x
> +
>   /*
>    * Initialized pointers to per-cpu variables needed for the boot
>    * processor need to use these macros to get the proper address
> @@ -516,11 +518,11 @@
>   	typeof(o2) __n2 = n2;						\
>   	typeof(o2) __dummy;						\
>   	alternative_io("call this_cpu_cmpxchg16b_emu\n\t" P6_NOP4,	\
> -		       "cmpxchg16b %%gs:(%%rsi)\n\tsetz %0\n\t",	\
> +		       "cmpxchg16b " __percpu_prefix "(%%rsi)\n\tsetz %0\n\t",	\
>   		       X86_FEATURE_CX16,				\
>   		       ASM_OUTPUT2("=a"(__ret), "=d"(__dummy)),		\
>   		       "S" (&pcp1), "b"(__n1), "c"(__n2),		\
> -		       "a"(__o1), "d"(__o2));				\
> +		       "a"(__o1), "d"(__o2) : "memory");		\
>   	__ret;								\
>   })
>
> Index: linux-2.6/arch/x86/lib/cmpxchg16b_emu.S
> ===================================================================
> --- linux-2.6.orig/arch/x86/lib/cmpxchg16b_emu.S	2011-03-26 20:43:57.384089004 -0500
> +++ linux-2.6/arch/x86/lib/cmpxchg16b_emu.S	2011-03-26 20:48:42.684088999 -0500
> @@ -10,6 +10,12 @@
>   #include<asm/frame.h>
>   #include<asm/dwarf2.h>
>
> +#ifdef CONFIG_SMP
> +#define SEG_PREFIX %gs:
> +#else
> +#define SEG_PREFIX
> +#endif
> +
>   .text
>
>   /*
> @@ -37,13 +43,13 @@
>   	pushf
>   	cli
>
> -	cmpq %gs:(%rsi), %rax
> +	cmpq SEG_PREFIX(%rsi), %rax
>   	jne not_same
> -	cmpq %gs:8(%rsi), %rdx
> +	cmpq SEG_PREFIX 8(%rsi), %rdx
>   	jne not_same
>
> -	movq %rbx, %gs:(%rsi)
> -	movq %rcx, %gs:8(%rsi)
> +	movq %rbx, SEG_PREFIX(%rsi)
> +	movq %rcx, SEG_PREFIX 8(%rsi)
>
>   	popf
>   	mov $1, %al

Tejun, does this look good to you as well? I think it should go through 
the percpu tree. It's needed to fix a boot crash with lockless SLUB 
fastpaths enabled.

                 Pekka

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