* Pekka Enberg <penberg@xxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxxxxx>
> >Signed-off-by: Christoph Lameter<cl@xxxxxxxxx>
> >
> > 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.
AFAICS Linus applied it already:
d7c3f8cee81f: percpu: Omit segment prefix in the UP case for cmpxchg_double
Thanks,
Ingo
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