lkml.org 
[lkml]   [2009]   [Apr]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] x86: percpu_to_op() misses memory and flags clobbers
Ingo Molnar a écrit :
> * Eric Dumazet <dada1@cosmosbay.com> wrote:
>
>> Jeremy Fitzhardinge a écrit :
>>> Eric Dumazet wrote:
>>>> While playing with new percpu_{read|write|add|sub} stuff in network tree,
>>>> I found x86 asm was a litle bit optimistic.
>>>>
>>>> We need to tell gcc that percpu_{write|add|sub|or|xor} are modyfing
>>>> memory and possibly eflags. We could add another parameter to
>>>> percpu_to_op()
>>>> to separate the plain "mov" case (not changing eflags),
>>>> but let keep it simple for the moment.
>>>>
>>> Did you observe an actual failure that this patch fixed?
>>>
>> Not in current tree, as we dont use yet percpu_xxxx() very much.
>>
>> If deployed for SNMP mibs with hundred of call sites,
>> can you guarantee it will work as is ?
>
> Do we "guarantee" it for you? No.
>
> Is it expected to work just fine? Yes.
>
> Are there any known bugs in this area? No.

Good to know. So I shut up. I am a jerk and should blindly trust
linux kernel, sorry.

>
> Will we fix it if it's demonstrated to be broken? Of course! :-)
>
> [ Btw., it's definitely cool that you will make heavy use for it for
> SNMP mib statistics - please share with us your experiences with
> the facilities - good or bad experiences alike! ]

I tried but I miss kind of an indirect percpu_add() function.

because of Net namespaces, mibs are dynamically allocated, and
current percpu_add() works on static percpu only (because of added
per_cpu__ prefix)

#define percpu_add(var, val) percpu_to_op("add", per_cpu__##var, val)

I tried adding :

#define dyn_percpu_add(var, val) percpu_to_op("add", var, val)

But I dont know it this is the plan ?
Should we get rid of "per_cpu__" prefix and use a special ELF section/
marker instead ?

I have a patch to add percpu_inc() and percpu_dec(), I am not
sure its worth it...

[PATCH] percpu: Adds percpu_inc() and percpu_dec()

Increments and decrements are quite common operations for SNMP mibs.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index aee103b..248be11 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -103,6 +103,29 @@ do { \
} \
} while (0)

+#define percpu_to_op0(op, var) \
+do { \
+ switch (sizeof(var)) { \
+ case 1: \
+ asm(op "b "__percpu_arg(0) \
+ : "+m" (var)); \
+ break; \
+ case 2: \
+ asm(op "w "__percpu_arg(0) \
+ : "+m" (var)); \
+ break; \
+ case 4: \
+ asm(op "l "__percpu_arg(0) \
+ : "+m" (var)); \
+ break; \
+ case 8: \
+ asm(op "q "__percpu_arg(0) \
+ : "+m" (var)); \
+ break; \
+ default: __bad_percpu_size(); \
+ } \
+} while (0)
+
#define percpu_from_op(op, var) \
({ \
typeof(var) ret__; \
@@ -139,6 +162,8 @@ do { \
#define percpu_and(var, val) percpu_to_op("and", per_cpu__##var, val)
#define percpu_or(var, val) percpu_to_op("or", per_cpu__##var, val)
#define percpu_xor(var, val) percpu_to_op("xor", per_cpu__##var, val)
+#define percpu_inc(var) percpu_to_op0("inc", per_cpu__##var)
+#define percpu_dec(var) percpu_to_op0("dec", per_cpu__##var)

/* This is not atomic against other CPUs -- CPU preemption needs to be off */
#define x86_test_and_clear_bit_percpu(bit, var) \
diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
index 00f45ff..c57357e 100644
--- a/include/asm-generic/percpu.h
+++ b/include/asm-generic/percpu.h
@@ -120,6 +120,14 @@ do { \
# define percpu_sub(var, val) __percpu_generic_to_op(var, (val), -=)
#endif

+#ifndef percpu_inc
+# define percpu_inc(var) do { percpu_add(var, 1); } while (0)
+#endif
+
+#ifndef percpu_dec
+# define percpu_dec(var) do { percpu_sub(var, 1); } while (0)
+#endif
+
#ifndef percpu_and
# define percpu_and(var, val) __percpu_generic_to_op(var, (val), &=)
#endif
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2009-04-01 19:17    [from the cache]
©2003-2011 Jasper Spaans